Judith Beck offers tips about how to increase willpower. First three ideas:
Specify your goal in behavioral terms. Not, “I want to be more physically fit,” but, “I want to go to the gym three times a week for 45 minutes.”
Make sure your goal is reasonable. If you have been sedentary, you may need to start off with a more modest exercise goal and increase your efforts as you go along.
Reflect on all the reasons you have to reach your goal. Think of as many reasons as you can and keep adding to the list.