I found out about this through one of the writing communities I frequent. It's excellent for anyone who works from home (anyone who works, really, but WAHMers will benefit most).
The Pomodoro Technique (simplified)
1. Set a time to 25 minutes
2. Work for 25 minutes straight
3. Take a 5 minutes break
4. Get back to work (lather, rinse, repeat)
So, I did this tonight. I wrote until my timer ticked off. Spent 5 minutes doing stretches, calisthenics and so forth. Then, back to work.
I was twice as productive as usual and slipped in 15 minutes of exercise. And I kept telling myself I didn't have time.