One word: Breathe.
This is both a literal and figurative lesson. In the literal sense, taking a deep breath and letting it out slo-o-o-wly is a good way of getting rid of momentary tension.
In the figurative sense, it's important to take a "breather" every once in a while. In the short term, just going outside for five minutes and enjoying the fresh air at night while watching the traffic go by can do wonders for my mood. Over the longer term, taking a day or two off if I've been tense or just plain busy tends to help somewhat. Despite the fact that it's sort of lonely, going home this weekend was kind of nice for the fact that when I'm there, the biggest thing I have to worry about is whether the cats have been fed.