Being in the midst of dealing with more stress than I've ever had to deal with, I'm currently trying new methods of stress relief. The first thing I do is pretty obvious - I try to eliminate the source of the stress.
When that isn't possible, I've been trying to work on a hobby. My current obsession is gardening. (Researching, planning, seed starting, etc.)
I also second the mindfulness meditation. I'm still pretty new to it, but I really like it!
For me, meditation works very well, but only if I do it daily and consistantly.
I am in the middle of my PhD, and last year I was stressing big time - worrying about whether or not I would find something novel, whether I would finish, whether I would complete on time etc. In the end I just had to take a step back and decide NOT to stress. Worrying about when I finish will not help me to finish faster (take that sentence out of context!).
So at the moment I have a huge milestone phd wise coming up, where I present my work so far, and complete a one hour verbal defence of my work and proposed work. I am someone who likes to have stuff finished early (no last minute all nighters for me), but my supervisor does not make this easy for me. Two weeks to go and I still haven't had feedback on my drafts etc. But I am choosing not to stress. Worrying about whether or not I have time to get everything done in time will NOT help me get it done any faster. I will do what I can do, and cannot possibly do any more than I can do.
I have found this 'decision' to not stress helps a lot. Obviously I still stress sometimes, but I don't tend to get caught up in sleepless vortexes of doom every night.