Also= start SMALL and SHORT. This is a reward of its own. When you try to create a habit of exercise, start doing only push ups or crunches for one or two minute. It seems to not be useful at all, but i guarantee it, you will feel better after you did it. Because it is so easy, very short and seems to be no strain on time, energy and circumstances, it will feel like you did really nothing, but got it done, and that is the important part. Do that one week, and you will muscles growing, and you will be stronger than one week before. You can do the same with learning stuff like math. Start with just one paragraph of a calculus introductory book, not more. You will be seriously smarter and more profecient in math after just a few weeks.