I've wondered about the same thing. When I'm depressed it's like I have no momentum. It's like I can't summon the strength to start moving out of bed or off the couch. It's like my body's spark has been extinguished.

Some things that help me sometimes (not all the time, I'll admit) are: (1) considering how I'll feel if I don't fulfill my commitment, and the relative cost of not doing so; (2) considering being honest with my friends about how I'm feeling, if that's the circumstance; (3) making a deal with myself (e.g., if I get out of bed now I'll treat myself to a nap after my appointment).

Good luck, and keep us updated on any new ideas you have.