It's important to find a way to measure how well you are keeping your commitment. This will help you see the progress you are making.
For example, each time you exercise, record what you do on paper. There are many styles of workout logs to choose from. At the very least, write down in your calendar or daily planner that you exercised on a given day. Doing this will help you see that you are succeeding with your new habit. Later in this book I'll again discuss self-monitoring and you'll see how important it is when boredom and lack of motivation become a barrier to exercise.
When you see that you've succeeded, reward yourself for a job well done. This type of feedback will help reinforce the positive behavior. I consistently preach this to my clients. It really helps.
The reward can come in many different ways. It can be massage therapy, a night out on the town, or buying that something you've always wanted. If you plan to reward yourself for a particular accomplishment, make sure you do it. And do it without regret!