The workout is a great thing to relieve stress.
Or playing with animals works wonders.
Personally, it is rare that I get in such a state now. I read Herbal's blog several months ago and a post was entitled "How to be Happy. Always" in which he detailed a small mental challenge that he did for 30 days in which every time he had a negative thought, he'd find a positive in it
For example; your car breaks down on a summers day. Instead of being irate that this has happened, he would say think "Now I have a chance to sit outside in the summer day, get some fresh air whilst waiting for the mechanic to come"
After doing this for a week or so, I found that it became a subconscious thing and that I very rarely thought completly negatively. Proof that it worked so well is that a girl who I've been on and off with, and am close friends with at the moment said "This is the first time I've ever seen you anything other than happy" when I was memorably down despite knowing her at least 11 months.
Try doing it, just for 30 days.