Aaron Swartz (co-author of the RSS specification, founder of reddit.com, Demand Progress, and generally a very cool guy) wrote an (you may want to not click on the link yet) article on productivity which had two illuminating ideas for me:
Time is not fungible. Telling yourself "instead of watching TV I should work on X" can be unrealistic if all you have the energy for at that moment is to watch TV.
We avoid tasks because they are hard/unpleasant, or if they are assigned.
I've spent a bunch of time trying to explore this and the best way I can describe it is that your brain puts up a sort of mental force field around a task. Ever play with two magnets? If you orient the magnets properly and try to push them towards each other, they'll repel fiercely. As you move them around, you can sort of feel out the edges of the magnetic field. And as you try to bring the magnets together, the field will push you back or off in another direction.
Assigned problems are problems you're told to work on. Numerous psychology experiments have found that when you try to "incentivize" people to do something, they're less likely to do it and do a worse job. External incentives, like rewards and punishments, kills what psychologists call your "intrinsic motivation" - your natural interest in the problem. (This is one of the most thoroughly replicated findings of social psychology - over 70 studies have found that rewards undermine interest in the task.) People's heads seem to have a deep avoidance of being told what to do.
The weird thing is that this phenomenon isn't just limited to other people - it even happens when you try to tell yourself what to do! If you say to yourself, "I should really work on X, that's the most important thing to do right now" then all of the sudden X becomes the toughest thing in the world to make yourself work on. But as soon as Y becomes the most important thing, the exact same X becomes much easier.
I'm very interested in the assigned tasks aspect - how do you get around it? Swartz has some ideas, but he admits "I've never been able to overcome this mental force field through sheer willpower". I'm curious what other people can come up with, before reading his article I've linked to in the first paragraph.