I have no experience with all-out depressions, but i do have a rather unstable mood sometimes, which can end up bordering depression for no reason at all. Although the following may seem naive and childish, i'm being quite sincere. Forcing a smile on your lips, regardless of how little it fits your mood, can actually make you happy again. This is because your mind is, by now, wired to connect smiling and happiness with each other, in a joyous cause-and-effect loop. In other words, you smile when you're happy, so you'll be happy when you smile. Give it a shot, you won't be wasting anything by doing it.
A bit of mind wandering can be helpful too, i've found. If you let your thoughts roam freely, you might eventually get tired of thinking about all the depressing shit, and suddenly your mind is frolicking in meadows and rainbows. This did happen to me more than once, but take it with a grain of salt, considering my mildly neurotic behaviour.
Also, what Knight of Fools said about improving skills and such? It works, a lot. About a year ago, i found myself being very down at just about every evening, possibly because i had nothing new to do. I then decided to pick up Game Maker, and start making a game i had in mind for a while. Not only did it feel great to exercise my creative muscles, i also got a lot of compliments from others. It doesn't matter if the compliments came from the internet, they're stilll people behind the nicknames. Although i never finished the game, my night-time depressions are few and far between now, because i know for sure that i'm not just a useless puppet, i can actually entertain others with what i do. And, my point with this: Do something creative! Making something - ANYTHING - feels great, and rightfully so. Write a story, a rant, ramble about something, anything goes. If you can't think of anything, ramble about how you feel. Getting it out will make you feel better.
Also, let me finish off by saying that regardless of what you think, people will gladly help you get out of a depression. You see all these helpful replies? People care about your well being. It's the same thing in real life. Losing a wonderful person like yourself to depression would be a loss for the world.