What keeps me going? Lots of choices.
I have several walking routes mapped out for work and home. I have several (and I mean several) work out DVDs ranging from Yoga to the Biggest Loser "Last Chance Workout" and lots of other stuff in between.
I have Wii Fit that My son and I like to do together. We are both a little competitive. We also play tennis at the local school and ride bikes.
The more choices you have available the easier it is to find something to keep you going when you don't feel like working out. It all about options.