Next time you feel tired in the middle of the day, set an alarm to wake you up in 15-20 minutes after you lie down. Short naps will make you feel more alert and refreshed. Longer naps (for me) don't seem to work as well and if they aren't timed right I will actually feel lethargic afterwards.
Another approach would be to vary your schedule so you aren't sedentary for long periods of time. Get some quick chores done, catch up on some phone calls, talk to your significant other, go for a walk, etc.