What motivates me is setting a challenging fitness goal... First it was complete Couch to 5K before the end of 2009. Now it's get an hour of exercise in a day (average) for 2010.
When I'm grumpy/moody/gloomy/frustrated/anxious/whatever, even though it's nearly impossible to drag myself out, knowing that I will post my results somewhere keeps me going. Sometimes I get up and go just because I am falling behind (which - after a weekend of slugging around, I am now way behind for my month goal. Yikes!)
Also, having some structure. I find I don't do well with the vague notion of "going to exercise." So I take classes that meet regularly. And I am going to go through the C25K running program again, at a faster pace. The organization of that 9-week program (3x/week) really worked well for me when I did it. After that, who knows ... I'm going to have to think up some other kind of structured routine.
And the advice "do [whatever] for just 15 minutes" that several folks mentioned REALLY works! I need to remember this more often ....
Hope it's all coming together for you!