Oh depression really sucks for motivation, doesn't it?
After about a week of just moping around doing nothing, it's pretty much the frustration at myself for not doing anything that "motivates" me to just get up and do what I have to do. But I realize that this isn't exactly a good thing, either, and that "getting over" the depression as much as you can till you're in a good/happy place again is the only thing that feels right and good.
So I guess the only way is to get yourself out of the funk. Music also works for me. I'm a rock fan so fast, lively rock songs work like a charm for me.
Tidying up your room/house works wonders. Once you see it all neat and orderly and you think to yourself, "I did that. I did something to improve myself," I guess it gives you a sense of pride and satisfaction.
Watching shows or movies you absolutely love (stay away from sad ones). They don't need to be comedies. Most of the time I pick from my favorite movies of all time.
Reading used to work for me, but these days it just feels lonely. It can depend on the book, though. If I find that it's a really good read, I just let it distract me for an hour or so then stop and see if I feel like doing the stuff I need to do. Not very effective most of the time, though, but it's worth a try.
This last one may be a bit weird, but it always seems to work for me. If I'm feeling depressed, suicidal, like self-injuring... I Google it. In your case, I'd read about depression. We've all probably read a lot a lot a lot about these things--enough to have the symptoms, treatments, etc. memorized but... I don't know. Even though I've read the same articles over and over again, reading about it for about thirty minutes or so helps getting me feeling back to normal (as normal as things can get, at least).