exercise
St John's wort
good nutrition
multivitamin
reassess your habits... are you feeling down about procrastinating? lack of motivation? social problems? try to address these; sometimes a small situation can make you feel really crappy on the inside.

stress from school eats me alive... but these things have helped me.
if that doesn't work you might want to see a good counselor or psychiatrist. be careful about the drugs, though - the side effects are often unpleasant, and they don't make the pain go away... they just dull it.