I read "The Power of Habit" and think that the science behind it is worth considering for people interested in this topic. It explains that habits such as exercise and dieting get easier with time when they are done as a consistent routine due to the nature of brain wiring, so eventually one might, for example, find themselves feeling as if they are just drifting through their workouts rather than forcing themselves to and through them. It's science-based rather than just a "you can do it!" kind of book and possibly worth checking out.