What shortnsweet said: exercise.
I have mild, "controllable" depression that comes periodically. The counsellings therapist I talked to recommend exercising at least 3 times a week. Something like a brisk walk for 15-20 min per session suffice. Exercise stimulates serotonin release. Exactly the same as what those antidepressive drugs do.
Another thing that might help is just finding somebody you can talk to about anything and everything. It feels better once you let it out.
Oh and try to smile and laugh often. Even if you don't feel like it... fake it. The subconscious mind takes cues from body movements.