Most of my mantras are quotes, but I'll be eliding the author (you can find him/her easily enough).

I have a truckload of them, but I'll try to make an informed selection.

Take a simple idea and take it seriously.

Often we know things that would good but we do not apply them, or apply them enough (more dakka!).

The unexamined life is not worth living.

I dread thinking what my life would have been if I did not have an introspective bend.

Your time is limited, so don’t waste it living someone else’s life.

I think about this when I get instagram-envy.

I have been impressed with the urgency of doing. Knowing is not enough; we must apply. Being willing is not enough; we must do.

One of many quotes about applying what we know.

Simplicity is the ultimate sophistication.

Worth pursuing; and it's also worth remembering that if something seems too simple, it may have taken a lot of work to get there.

Research your own experience. Absorb what is useful, reject what is useless and add what is essentially your own.

Crucially, there is something that is essentially your own.

Above all, try something.

There is one rule, above all others, for being a man. Whatever comes, face it on your feet.

Don't give up, don't let the circumstances do the choosing for you.

To be free is nothing, to become free is everything.

Everything is more meaningful when it's been wrangled from life's cold fingers. We're all born in invisible chains we must rid ourselves of.

What is the mark of liberation? No longer being ashamed in front of oneself.

Shame is the motivation-killer.

Whenever there is any doubt, there is no doubt.

I have a tendency to give many things or people the benefit of the doubt. Sometimes, and in some provinces, you don't want to do that.

"Don't screw future self" is one that has served me well for more than a decade.

El mayarah (stronger together) and Live by the sword, die by the sword.

It's a reminder that having friends is a valid strategy. Surrounding yourself with people doesn't necessarily make you weaker, and it's possible to form a group that's more than the sum of its parts. Cooperation can assimilate chaos if the conditions are right; it's not an immutable axiom that chaos triumphs over order. A group is not necessarily dead weight, and relationships are not necessarily weak points, if you pick the right people to connect to.

The first one is from Supergirl and the second from SlateStarCodex.

One of my favorite mantras is "A citizen has the courage to make the safety of the human race their personal responsibility" from the movie Starship Troopers. While a lot of the meaning is caught up in the movie's setting, the meaning that I personally draw from it is that an important part of living in the world is working hard to make the world a better place, and not assuming someone else will do it for you.

"When in doubt, go meta". Thanks to my friend Nadia for quoting it often enough for it to have found a place deep within my brain. May not be the perfect mantra, but it is something that occurs to me frequently and almost always seems yet again unexpectedly useful.

The good ol' Litany of Gendlin:

"What is true is already so. Owning up to it doesn’t make it worse."

I’ll play my part to do what’s right It’s ok, to be ok :) Im already at the center of love It is always time to listen to the song There are giants in the sky!

I'm still determining what sort of mantra would help me work towards goals with long time-horizons but for the time being I've got a few quotes:

It is a sobering thought that when Mozart was my age, he had been dead for two years.

It is a shame for a man to grow old without seeing the beauty and strength of which his body is capable.

The one who puts the finishing touches on their life each day is never short of time.

what would you gain from everlasting remembrance? Absolutely nothing. [So what is left worth living for? This alone: justice in thought, goodness in action, speech that cannot deceive, and a disposition glad of whatever comes, welcoming it as necessary, as familiar, as flowing from the same source and fountain as yourself.]

The future is already here — it’s just not very evenly distributed.

I've never consciously thought in terms of mantras as far as I know, so there's probably a good answer in my brain I'm failing to recollect.
This sounds like a good way of making a thought easy to recall.

Not a series of magic words, but I regularly think along the lines of "it can be done." That people can accomplish amazing things with time and effort. It is not a question of if I can, but if it is worth my unfortunately limited time.

"If I was born in their body, and lived their life, I would make the same choice." - If you believe human behavior is predictable like any other physical system, this lets you feel some empathy for people you don't understand.

"That sounds like the plan of an ordinary man" - Jonathan Coulton.
Reminds me that I have high standards for myself, and I'm not going to meet them by taking the easy options.

Not words I live by, but I like the energy behind "If you can't beat 'em, make 'em bleed like pigs." - Mountain Goats
The sense of "all hope is lost? Then push harder."

 

Here's some mantras I have:

That which you are aware of, you are free from.

And some variation of:

Truth comes knocking. You say "go away, I'm looking for the truth." It goes away, puzzling.

The above I rediscovered recently through reading Zen and the Art of Motorcycle Maintenance.

This was a more embarrassing question than I was expecting... well, here it goes.

Who the hell do you think we are?

Do the impossible, break the unbreakable 

Row, row, fight the power! 

Kick reason to the curb and do the impossible.

These three are straight from Gurren Lagann. I use them often as mental rally cries when I feel I'm at loss for hope or about to give up on something.

The first is a vague "don't give up and persevere" mostly for getting grit in the moment.

The second is more for my long term plans that are a long way from my reach (I know it's a misquote, but I like it more expressed this way.

The third I always interpreted as "kick common sense to the curb and think a way to do the impossible", I use it  for situations where I see no way of winning and have to make me think a way anyway.

Shut up and just do the impossible.

From HPMoR and Lesswrong, when I have to solve something I'm certain I can't, and the price for failure is high. Most of times I used I managed to at least make the situation better. 

I can do anything if I study hard enough!

I can do anything if I think hard enough!

From HPMoR, courtesy of general Sunshine. The second is more or less as the one above, the first one is for pushing through plans or reach goals that would ask me to study for a long time.

Are you making an extraordinary effort?

Are you doing everything you can?

Less pleasant than the ones above. I use these two to both step up my game and take ideas seriously. 

The first one is from Lesswrong, but I had been using the second one from a while already, so they got paired up. I got the second one from reading something about Greta Thumberg, and it hit me that it was the first "ordinary" person I saw described in details as one that was actually taking the climate change issue seriously and  behaving as she believed that much was at risk. 

(To clarify: I don't think she's the only one doing so, but her behaviour had struck me as impressively more coherent than what one would usually expect and left an impression)

They seem to put on me a lot of stress though, since they require me feeling like everything depends just on my ability of breaking my limits, giving my 100% effort, and then somehow reach past that an order of magnitude more. I'm using it only on my current life goal. So far my results ramped up.

Are you trying to argue you are right or to understand where the truth is?

This is my "rationality mindset, on!" mantra, it seems to be pretty effective on stopping certain bias when they activate and make me look at a question with the right mentality. I've often changed my mind and ideas when I used it, so I think it works pretty well.

Remember you can choose not to care.

Remember you can regulate your emotions.

I use these when I'm feeling bad for something I can't change or that I don't think I should be feeling bad about, or when I'm in the grip of anger or some other emotional state that's hindering me. 

I'm not 100% sure it's the healthiest thing I could say to myself, but it did got me through a light depression when I was a teenager and stabilised my mood a lot, so it stuck.

"When will you learn that your actions have consequences." It's from a terrible meme, but boy does it have a way of being relevant. 

"It's okay to care" -  I repeat this mantra when I notice I'm stopping myself from caring.

I was a nerd and a bit of a social outcast growing up. Nothing too extreme, but enough to feel different and lonely at times. I was always interested and obsessed with things but when I'd go to share it I'd usually get a "who cares" kind of response. Eventually, this morphed into an internal edict: "It is not cool to care". Liking something makes you vulnerable and I liked a lot of things.

I ended up going to a good college. Lots of math, art, philosophy, and film nerds - I met a lot of people who cared and it was great, but there was also a lot of "care-shaming" in a different direction. Namely, if you liked something that wasn't nerd-approved or subversive enough it was very much not cool.

Sometimes I just want to watch baseball and unironically care about it. I had a lot of moments like "Yes yes I know that Chomsky said sports are just 'training in irrational jingoism' but please, just let me go to a bar and get excited about the Super Bowl without having to justify it." Let me care in peace.

When someone cares about something harmless, I do whatever I can to let them have it, even if I don't care about it.

Just a restatement of "We ... do these other things not because they are easy, but because they are difficult," which for me is, "It's hard. Do it anyway." When inattention and repeated failure in grad school turned to constant anxiety turned to depression, I often found myself not doing things because they were hard, and in moments of clarity, that would rankle deeply, but I'd be unable to break past the "it's hard" barrier. Then, I found myself taking a firm, mentoring hand with friends of mine - one I never turned on myself. So, I began internalizing and repeating, "It's hard. Do it anyway," whenever the only reason I could give myself to not do something was that it was "hard." I liked it as a phrasing that brooked no rebellion, but acknowledged that yes, the task was hard. There are times, still, on particularly bad days, when "hard" is even as simple as doing the dishes, but this mantra does get them done.

The motto I try to live my life by is "Live a life that would make you happy, if others lived as you live" - it's like the golden rule, but even better. Obviously, it's influenced by a lot of thinking about decision theory, both as discussed here on LW, and in my own thinking about how to live life. I do probably have a lot to improve in terms of actually integrating my behavior with this rule.

Some other mantras I like:

• From "How To Win Friends and Influence People" by Dale Carnegie (a very handy guide to not being an asshole): "The only reason you aren't a snake yourself, is because your parents were not snakes"

• User Dagon on LW once wrote "If you aren't taking actions contrary to incentives, you aren't making moral choices"

• The future only exists as expectations based on past experiences

• Even if you are twice as good a medic as the typical medic, if you are ten times as good at firefighting, you should be fighting fires, not doing medicine. (This is Adam Smith restated)

And, I guess I'll add a quote from a mathematician, just for our host Alkjash: "Sometimes general questions can be cleaner than specific questions, and easier to analyze" - Tim Gowers

It's like a finger pointing away to the moon. Don’t concentrate on the finger, or you will miss all that heavenly glory.

If we focus too hard on the minutia of our daily tasks, we can forget the beauty of their ultimate purpose, the vision which they advance.

This doesn’t scan like a good mantra, but after you hear Bruce say it...

Jonathan Haidt's writing is rife with these. One that has stuck in my mind is "Intuitions come first, strategic reasoning second."

Failure is acceptable; failing to try is not.

"Pain is inevitable but suffering is optional"

I already know that I don't enjoy pain/like that person/approve of how someone treated me/like being tired, so why do I need to suffer over it? Why not just correct the error if it is correctable, let it be if not, and then move on?

I like the mantra, "If we choose to give more effort today, then we are sure to go beyond our past mistakes." This mantra is on my desktop screen.

Most of the sayings that could become mantras are rarely used, but stored and sometimes reviewed.

Loosing hurts if it's a loss I can survive.

For me it means that any feeling of pain is an evidence of my continued life, helps to direct attention from past into action for my current and future self.

Helpful after grieving period.

The rules of the game are quite simple, but take a lifetime to master.

I take it to mean that most basic underlying rules for living are simple, something like "You can do anything" said in sarcastic tone.

But not all possible actions are helpful, and it takes experience and work to learn to live in a way that would be prosperous.

In addition it helps to notice that systems and rules devised by society are not the "reality" and could be changed, but also that they are complex and intricate systems that are hard to understand and worthy of understanding.

If I keep a green bough in my heart, the singing bird will come.

To me in means: "Even through the darkest times I need to save the seed of good, if I want goodness to thrive again" - to not loose all hope.

In reality I don't literally believe that some good outcome will inevitably magically happen if I manage to guard and save a small amount of hope and tenderness, but that helped me out several times.