The difference between good and bad depression is a target. If a specific person (NOT yourself) gets you down, it is possible to convert this depression into a rational or otherwise hatred of this person/thing. Or, if you're more the constructive type, you can do something about it and rectify the dispute. Examples include apologising or offering gifts to a person, or moving the chair you've just stubbed your toe on. (However, if you've never tried it before, trust me: there's definitely something to be said for bashing chairs against nice, solid trees. On the infrequent occasions I've tried it, I haven't yet found a problem it couldn't solve ;·)
To rectify depression, you basically need to fix the problem. When you don't know what's making you glum, it ain't good. This is where Bad Depression kicks in. You either can't see why you feel the way you do, or you can see the cause but can't do a thing about it. One way to get here quickly is thinking too much about stupid things you've done, how you've hurt friends, or how you could have done x to prevent y but didn't.
I hate to admit this, since it doesn't seem like the logical way of solving a problem. But taking a walk, preferably up a mountain, helps. It does. Take karate if it's more convenient/exciting (I do both.) But unless you do this regularly, or perhaps even if you do, it is only a temporary solution...unless inactivity was the cause of your depression, which is entirely possible.
People cheering you up will, similarly, only have a temporary effect. (I'm only talking "there, there, poor you" style cheering.) Unless you squish it down into a corner of your brain (not recommended!), you're gonna have to deal with this depression to make it go away. Change your way of thinking. (Other people can certainly help with this.)
Focus yourself. Find a target. Be reasonable, find a good reason why dealing with this person/thing can make the world a better place (for you). If you're going for the hatred approach, you certainly don't need a good reason, just try not to develop an undying hatred of your boss, or anybody capable of (and willing to) kick your ass the next time you give 'em the ole Death Stare.
Though I generally use plastic chairs, in the event of a TOTALLY disastrous situation arising, I also have easy access to a nice metallic chair, which will surely help me in my quest to knock down that infernal tree...root of all evil, that tree...