Not in law school yet, but I've had some pretty high-stress jobs before, so I'll throw in my $.02:
Agreed with all who emphasized exercise. For me the best thing (especially for stress) is a mix of running/elipticals and yoga. And even though I don't do it every day anymore because I'm lazy and love to sleep, AM yoga first thing in the morning really does put me in a better mood for the rest of the day. (Side note: I've been living abroad since September, and back in the States I had an awesome free gym at my office building. I just finally started working out here, and it has made a world of difference!)
I definitely do not have the best diet in the world, but I always try to eat a lot of veggies, even when the rest of my diet sucks. Also, since I'm a vegetarian, I have to make sure I'm getting protein, etc. And I take iron and vitamin B-12 supplements. I definitely noticed feeling more energetic when I started taking those, too.
I'm also a big proponent of having a glass of red wine with dinner to help wind down at the end of a long day.