HOWTO: Be more productive by Aaron Swartz:
"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.
So the secret to getting yourself to do something is not to convince yourself you have to do it, but to convince yourself that it's fun. And if it isn't, then you need to make it fun."