I want to share this method I use sometimes to stay focused on my tasks, earn rewards from them, and build up a positive feedback-loop to do more difficult things. It's nothing new and has probably been written about a few times. I have been using it subconsciously for years, and wanted to do an explicit representation for future use. If this sounds completely wrong to you, please ignore it or tell me in the comments.

It should go without mentioning that this is just one part of a well-tuned system. It works because other parts work and support it. If supportive systems are wired differently or broken, this approach may not work at all.

What you need

the ability to motivate yourself to some degree
some preparation, or else that you have a mental list of tasks that can be done
the ability to complete simple or moderately difficult tasks when you are already motivated (Side-note: Forcing yourself to do things when you really, really don't want to might be effective one or twice. But in the long run, it's going to build up an even stronger aversion. Then making yourself do the thing will be even more difficult. How to change your feelings about a task is not the main topic of this post.)

How it works

Step 1: Induce happiness. Make yourself feel confident and hopeful.
Step 2: Choose the (simple to moderately difficult) action you want to complete.
Step 3: Complete the action and earn reward for this! With the "evidence" of being able to complete actions, stir up your confidence of being able to complete tasks.
Step 4: Choose the next stack of actions. They should be moderately difficult and involve clear steps to the solution (no 'meta-actions'). Begin them immediately, before your confidence and motivation fades; in doing so, focus on the short-term future when you are going to feel accomplished about having completed them. Use your recently activated confidence for this. Don't focus on your potential dislike of them or any other feelings of avoidance. If they crop up regardless, ignore them and tell them that they are going to be defeated soon, so they should just leave. (I know how this sounds. But if you are anything like me, treating your mind like a dog to be trained can be really helpful in getting it to do things!)
Step 5: Relax from having successfully completed a stack of necessary and useful actions. Bask in the reward, but don't linger more than 10 minutes. Even if you are slightly exhausted, this is not the time to stop! Focus on building your resolve to tackle a more difficult action next; one that you know you can complete, but may have to work harder for.
Step 6: Make sure to possess the necessary energy to do this more difficult task. Eat or drink something small and healthy if you don't; take a short (5 to 15 min) nap.
Step 7: Sit down in a clear and organised workspace. Be determined to do this! Don't stop until you are finished. Persist through exhaustion; this is a sign of working hard, not failure. When you are done, take the time to clean up your workspace.
Step 8: Rest a lot! Go to sleep or take a nap, eat something, read a light novel. Well done! You have completed your goal!

Caution! Do not use this to work yourself to exhaustion over time. This is meant to help in keeping up a healthy work-mentality; don't use it to trick your body or mind into giving more than it has. Take steps to make sure this doesn't happen; perhaps set up a reminder for some weeks later, checking that your habits don't stray into forbidden territory. The danger lies in not noticing until it is to late. Be prepared!

How to reward yourself

This might be really different for different people. I build up some habits over the years where, after completing some chosen task or thingy, I would internally congratulate myself, focus on the positive feelings this evoked, etc.

It might take some experimentation. Physical rewards, like a pleasant sound and light effect, a 'Well done!' stamp on a paper (humans are weird, but if it produces the desired results...), can also be effective. This works for children, pets and games, which is why I started using it.

These small rewards don't really matter at all, of course. They are just tools to build up the desired habits. Eventually, when you are working on the things that are important to you and making progress, that may become a reward of its own.