How would you advise someone to train their willpower?
I think it’s mind over matter. As Sun Tzu said, “He who conquers others is strong. He who conquers himself is mighty”. Buy a self help book might do the trick like “Awaken The Giant From Within” by Tony Robbins.
Give yourself the best chance to make the better decision when the time comes. That means planning out meals, workouts, and being proactive rather than reactive. Taking an active role in your health will make it easier to exercise will power over the things you can control.
Do something that is so hard that it make you want to quit. At that moment, when you absolutely KNOW that you can’t possible do it anymore – that’s when you’ve just begun.