Become, and stay, motivated to exercise.
Intrinsic motivation—that is, motivation from within—will last a lot longer than any external motivation (such as a reward system). The above steps will help you cultivate intrinsic motivation to exercise.
Set your own goals.
Create an exercise plan that is tailored to your needs and fitness levels. You can draw inspiration from standardized exercise plans, or you can consult a fitness trainer for advice, but you must make the final calls yourself.
Discover fun ways to exercise.
Going to the gym and hitting the treadmill for 30 minutes a day can get boring fast. Instead, try going for a jog in the park, or play an informal game of tennis with some friends.
Pick an activity in which you can improve over time.
You might be extremely unfit right now, but after even a week of practicing your activity, you will notice improvements. These improvements will then inspire intrinsic motivation in you, so you can get better and better!