Chris Wanstrath delivers a fascinating keynote address on why you should make the time in your life for some sort of side project. He specifically addresses computer programmers, but the advice could apply to anyone.
And how does he recommend that you motivate yourself to work on the side project? By tracking your results:
"Every time you work on your side project, mark a big X through that day on a calendar. Eventually you'll have a nice line of Xs. Missing an X will be torture -- it'll mess up your beautiful streak. The goal is to maintain the streak, even if you don't think you have any ideas for the day. The best way to overcome writer's block is to write, after all."
This probably isn't the first time you've heard this advice, and it probably won't be the last. But I want to stress that it's worked for me, too. I now loathe to mark down that I procrastinated the previous day, and I love to mark down that I spent the time in particularly productive ways, like working on a long-term project.
If you're thinking of tracking your results (on anything!), I recommend Google Documents, so that you can do it anywhere and don't need to worry about losing a piece of paper. We'll see if it works for Jer and his 25 book challenge.