A) Go outside. Don't just walk into another room and start watching TV.
B) Walk around for at least 10-15 minutes.
C) Even better, go for a jog, play some basketball or tennis, walk your dog, or participate in some kind of activity that involves movement.
I find doing these things, even if you feel tired from working in front of the computer all day, will actually re-energize you and make you feel much better.