As a Blake's 7 fan, I should like to state categorically that I've never touched Zen in my life, and nor would I. Now Orac, on the other hand...
Seriously, when I'm not going through a depressed period, I find it very easy to stay calm and relaxed, just by...well, I couldn't really say. It's just something I've taught myself over the years. Things that can stress you out fall into two categories - things you can do something about, and things you can't do anything about. Anything in the latter category, I try to just ignore and not let them get to me, whilst anything in the former category, I just try to do whatever I need to do.
Doesn't always work, but it's definitely a good habit to get into.