What I personally do when I'm feeling down:
1) Eat healthy and and exercise to improve your physical appearance. This is the big one, the key.
2) Get some things done you've been meaning to do but haven't.
3) Read some books, expand your knowledge horizon.
4) Find a hobby you're good at. I'm always looking for this and so far haven't found anything that I'm really good at yet, but there's bound to be something out there.
5) Talk to people, smile. Make yourself go through a whole day without saying or thinking anything negative.
6) Refer to 1) and do it again, over and over.