I've wondered about this as well, since wrote an essay on reinforcement schedules inherent in pinball games. I use the pomodoro technique, which fancy as it sounds, is just a timer that lets you check email and blogs after doing 'work' for 20/25 minutes. If you use a bit of software to manage it, it provides its own reinforcement to continue. ("you have completed x number of chunkc of work today/this week...").