The trick is this: before you exercise, think about how awesome you will feel afterwards. Think about the increased confidence, decreased anxiety, and feelings of accomplishment.
This simple action will actually help increase the likelihood that you experience those positive outcomes from your exercise. It becomes a self-fulfilling prophecy. Remember how great you feel. Really experience all the positive effects of exercise. Think about those feelings before you exercise next time. That will then feed in to your expectations for the next session, building on itself. Thus the "upward spiral."
You can even tailor this to your specific workout. Going for a personal record? Think about how it felt the last time you were able to achieve a goal like that. Feeling too tired to exercise? Think about how much energy you'll have after you get done.
Pay attention to every sensation-- the memory will be the fuel you need to get off the couch for the next workout.