I agree on physical exertion helping-- Martial arts has always helped a lot for me. However, I'm only involved in that off and on, due to time and money constraints. Most any kind of exercise will do, though, as long as it's enjoyable and leaves me feeling tired, sore, and accomplished.
That's more of a preventative measure than anything, though, so it needs to be regular enough to keep me from getting to the point where I feel like hurting myself. It doesn't work to calm me back down once I've already worked myself up to the point of wanting to hurt myself.
...sometimes I can calm myself back down by going for a long walk with loud music, though, because that combines physical movement with distraction and gets me away from my house (which is usually the source of my frustration as well as the easiest place for me to find things to hurt myself). As long as I'm able to take a walk, I'm usually okay. The problem arises when for whatever reason, walking is not an option. (Such as in the middle of the night, or while I'm in the middle of something that I really can't just stop.)