| When I go through periods where motivation just isn't there in the quantity I'd like (and I'm kind of going through one of those periods now), there are a few things I try to do:
1. I remind myself that I always feel better after I exercise. Always. So while some days it might seem like the hardest thing in the world to get my butt out of my chair, I know that after I exercise I won't remember what the big deal was about. At least until the next day . . .
2. If I'm really resisting, I tell myself that doing something is better than doing nothing, so it's OK if I cut back on what I usually would do. So if my typical goal would be to jog for 45 minutes or an hour, I'll take a 25-30 minute walk and say that's fine for today.
3. If I really, truly, just can't stand the idea of putting on the gym shoes, then OK, I won't. But I give myself a deadline: it's OK if I don't exercise today, or tomorrow, but I absolutely must do something -- anything -- two days (or whenever) from now. Usually if I give myself a break of a couple of days (or three), that's all I need to get back on track.
But the main thing to remember is to mix it up, move around (however you decide to do that), and never forget that it really does make you feel better.  | 
