From Never Say Diet

Buy a calendar just for your workout schedule. For the next two weeks, pencil in exactly what you're going to do for a workout, such as "30-minute bike ride after work." Don't forget to pencil in a reward at the end of two weeks if you successfully complete all your workouts. Try to make it related to something other than food. Maybe you'll get a manicure with a friend or buy a new water bottle. Give yourself a bigger reward if you fit in all your workouts for the entire month.