If you have a desk job, you're supposed to take breaks every once in awhile, especially if you do RSI-prone work. Those breaks are a great time to sneak in a little fun, maybe some balance training, grease the groove for pull-ups stretch, etc.
I was always forgetting to take my breaks until I found RemindMe, a free service that will send you IMs at specified times and days. You can set them to repeat every weekday, which is perfect. You can also IM their bot for one-off reminders, or to repeat the most recent reminder some time in the future. Very handy.
Update: Now I use Workrave.
