I used to get insanely depressed too, but these days I have been very stable.
The secret to that is getting an adorable puppy. Seriously, its the best thing that ever happened to me.
Ora more instantaneous solution, go buy some drinks, borrow movies and music that makes you sad, and cry your heart out.
If you cant beat it, join it!
My personal favourite is Evangelion anime and movie, Iguana's daughter full season, Prozac Nation, Requiem for a Dream and morbid music.
OR, if you desparately need to be happy, go to a stand up comedy site and watch videos.
Laughing always works.