For running/jogging, a good trick for when you really don't feel like it is to put running clothes on, go outside, and then just start walking. If you still don't feel like running, you can just go for a walk instead, but many people will actually want to run after a bit--the bottleneck is getting out the door.

It's the same for other kinds of exercise. If you're dreading lifting weights, start by just doing some sets of really light reps. If you still feel bad, you can just stop after that, but you'll probably often find at that point that you actually feel like doing more.

Weirdly, I seem to often have my best runs/workouts with this approach, when I started out feeling like crap. Motivation is a strange thing.