Fad diets fizzle when you run out of willpower. The solution isn't to get more willpower, instead it's to use what you have in focused bursts to build a system so that you won't relapse when your willpower fades again.
Any scheme that relies on the continuous application of willpower simply won't work. Human willpower is limited, and the easy way out has a tug that is hard to escape.
Instead, magnify the effect of the willpower by changing the system. If you want to eat better, rather than using that willpower to eat something healthy right now, use the willpower in a focused burst to make healthy lunches for all week. That makes meeting your goal the easy thing to do. Just grab the lunch and go. You've done all the work. You know it's good for you, but more importantly, it sets up the conditions for success. You're more likely to stick with it.
Suppose you want to reduce how much you spend on gasoline. You can't control the cost of fuel, so instead you need to control your consumption. Trading in your guzzler for a sipper would be step in the right direction, but then when gas prices go up you're still stuck paying for gas.
Even if you buy the sipper you will still be stuck with the same commute that was taking up your time and racking up your gas bill.
A more radical solution would involve moving somewhere where you don't need to drive. You'd eliminate the gas costs, as well as all the other costs of owning a vehicle.
Another perk to going car-free is that you'd be able to walk or bike to work, and get the health benefits of that incidental physical activity. It doesn't feel like exercising when it's simply how you get where you're going, but the benefits are the same.
Whatever your goals, use your moments of willpower to set up a system where accomplishing your goals is the default option and achieve them with ease.