How do you motivate yourself to exercise when you can't even get out of bed?

This is kind of in relation to jmah’s earlier question. I really need to start exercising again to bring myself out of this morbid cycle of depression I’ve been in. Does anyone have any tips of how to take that first step out of bed? Any mind games or motivation tools they do with themselves when even standing up is a challenge?
I used to run cross country so I do know that amazing, unparalleled feeling after a long run—endorphins pumping through you. But even when I try to recreate that for myself, I still can’t peel myself out of bed. So, any advice would be greatly appreciated.
I’m taking medication, too, just for the record