I certainly do my sit-ups too, but throughout the work day I keep the following two rules (that help me stay in shape, and keep moving even while working predominantly under sedentary conditions:
Regardless of the fact that most of my work is done in front of a computer screen, I do have to get up during the day (to grab a coffee, send a fax, send my mail, etc, etc). I’ve made it a rule to never sit back down in my chair unless I’ve done a 10-20 squats. Adding all squats up at the end of the day, you arrive at an impressive number of them, and body likes it.
Determine a regularly repeating thing you do throughout the day (be it posting a tweet, checking your e-mail, or even using a restroom), and make it a rule to do a number of push-ups after every occurrence of it. If necessary, take a day off after the day you’ve been doing your push-ups. Increase the number of “per set” push-ups on the following day.
Also, remember to drink plenty of water, go to a gym or a swimming pool at least 2-3 times a week, and get enough sleep to feel rested.
Stay healthy, folks! It’ll keep you productive for longer time.