Write down your strongest desire in terms of actionable goals. Instead of writing "I will be a size 6," write "I will work out for 30 minutes at least 3 times a week, and I will eat a healthy breakfast every day." Use your willpower to focus on steps in your control that you can commit to taking every day instead of an result that may be out of your control.
Be realistic. If your desire is to fit into a dress three sizes too small for you in a week, you're setting yourself up for failure no matter how strong your willpower, because such a task would be dangerous, if not impossible. When you fail, you're likely to altogether give up your goal of being thinner. Your goals should be challenging but reasonable, and able to be broken down into small steps that aren't too daunting.
Strengthen your willpower by listing the reasons for wanting this goal. Each reason should be deeply meaningful, and looking over this list should inspire you to accomplish what you need to when you feel your willpower is lagging. If what you desire is a different career, reasons like increased compensation or better job satisfaction will give you a boost of motivation when you don't feel like sending out that tenth resume.
Take a few minutes every day to visualize yourself having reached your goal. This motivational exercise will increase your willpower by making your goal seem attainable. When top athletes win competitions, they are sometimes asked if they are surprised they won, and they often respond that they are not surprised at all, because they envisioned themselves winning over and over in their mind. Something that seems within reach is easier to achieve.
Silence the negative voice in your head. This voice tells you that you cannot do something, or makes excuses. One way is through positive affirmations, which are daily repetitions of a statement that will build your confidence, like "I will exercise to strengthen my body and make myself feel good."
Come up with creative solutions to problems that sap your willpower. For example, If you find yourself regularly tempted to make impulse purchases that will ruin your desire for a debt-free life, block promotional emails from favorite shopping sites. Anticipating future roadblocks will make things as easy for yourself as possible and therefore make your success more likely.