There seems to be a ton of LessWrong articles on productivity and procrastination. I suspect that reading them is just another form of procrastination, but since you asked... (At least I sorted them by highest karma, so the ones considered most insightful by the community are on the top.)
With books and seminars on productivity, there seems to be an inherent conflict of interest: the authors profit most if you keep buying more and more of their books and seminars; but that is the opposite of actual productivity.
Here are some random thoughts in random order:
Keep it simple; the core ideas of GTD are important, obsessing over details is not. Make notes of what you want to do. (Keeping things in your head is a waste of your processing power.) Except for things that literally take a minute to do and you can do them right now; just do them right now. Outline the list of steps in a project, so it doesn't happen that you do the step 1 and then forget completely about doing the step 2. Set up reminders for things to be checked later, or for things to be done when you are at a computer, when you are in the city, when you are with someone, etc. That's all; the extra energy you spend setting up the system would be better spent actually doing the tasks. Because at the end of the day, the system won't do the tasks for you.
Consider the effects of trivial inconvenience. For example, I make my TODO lists on paper, not on computer or phone, because that way they are accessible even when the computer is turned off, and writing on paper is faster (I don't need to start the application, set up the font size and paragraph indentation...) Also, the computer and phone tempt me to start doing something else. Buy a heap of paper, and three reserve pens, so that you are never stopped by a lack of pen or paper.
(For example, cooking became much easier for me when I wrote the recipes on small pieces of paper I keep in kitchen. I can browse the papers and choose the one I feel good about. I can put the papers in my pocket when I go shopping, and use them as a shopping list. I can put the paper on a table as a reminder for tomorrow. Paper is a fantastic invention: it's like a computer application that you didn't have to code.)
Admit your addictions. Instead of reading yet another popular-psychological book or article on "akrasia" maybe you should just stop browsing the f***ing social networks. Yes, you are probably an addict, and whatever you tell me is just a clever attempt to avoid having to address the actual problem. (How do I know? Well, we are debating on internet about productivity, so the probability is very high...) If you have any social network applications on your smartphone, remove them immediately!!!
The most difficult thing with work is to actually start. Afterwards, it often gets surprisingly easier. But before you start, it sometimes feels really scary. A useful trick is to commit to exclusively focus on the work for the following five minutes. That means, you promise yourself that for the following five minutes, you will not be doing or thinking about anything unrelated to the work. If is means looking at the paper and not knowing what to do, so be it... you still promise to not start doing anything else until the five minutes pass. (Yes, watching the clock also counts as "doing something else"; set the alarm.) After the five minutes, you are free to stop... if you want to. But if it already got easier, you will probably want to continue.
When the task feels big, divide it into smaller parts. When I work at computer, I sometimes make notes: "do this, then do that"... then I split the points into subpoints... and at some moment the sub-subpoints get so specific that doing them is trivial.
Sometimes people do less horrible tasks to avoid doing more horrible tasks. Hey, it's much better than doing nothing! But at some point you have to address the problem.
Give yourself some time to relax without feeling guilty. No matter how unproductive you are, beating yourself up 24 hours a day is not going to make it better. (No, do not spend that time reading web. Lay on the sofa, or take a walk...) Actually, be nice to yourself. (Read Don't Shoot the Dog to fully understand why.)
Make notes about the math stuff. Different people use different systems; my current favorite is cherrytree. Don't think about it too much; start making notes now, you can refactor them later; any notes is better than no notes. (Generally, doing anything now and reflecting on it and improving it later usually beats long planning up front.)
Often things are easier when you do them with someone else. Talking to people helps to organize your thoughts. (Though you can also talk to a rubber duck.) If you like the person you are talking to, you associate your work with something pleasant. But it also helps you to stay focused on the topic during the talk.
Body and mind are connected. Sleep and exercise help you think better.
Many productivity techniques have this weird effect that they work like magic for a week or two, and then stop working completely. Learning another technique means getting another week. (This is great for people who sell productivity advice; you will soon come back and pay for more.) The underlying reason is emotional: some part of you actually doesn't want the productivity to get fixed. I don't have a simple solution for this. "Listen to yourself" is much easier said than done, but take some time to refect on how you feel about things, and be nice to yourself. Also, the important thing is not to never fail, but to always get up and try again.