I used to be a runner in highschool and went to state for diving. So I had to have pretty much the whole package to compete successfully.
I hit college and it went downhill fast since I didn't have those sports anymore to keep me going.
My motivation? When you aren't able to do things that you once were able to do. Benching over to tie your shoes and having issues because your gut gets in the way. Having to take a breather when you run to the end of the driveway to get the mail.
Then, the ultimate slap in the face, which got me into P90X, when the fiance goes "maybe you should start working out..."

When that happens, you know its time to do something and get off your bum and start working out. You know you can do it since you did it before, its just DOING IT.
BRING IT.