I have a feeling that "discipline", that is, getting yourself to do things by sheer force of will, only works in short bursts. For long periods, you need create a system for yourself that takes your own weakness into account. For example, for a long time I tended to exercise in intermittent bursts. I would do it for a few weeks, then catch a cold or something and fail to pick it up again afterword. The solution to this problem was to create a schedule for myself. I wake up, eat breakfast, check the internet, work for two hours, exercise, eat lunch, etc.

Another important thing is strong principles. That is, you have to have clear, logical reasons for making the sacrifices necessary to achieve your goals. Every time you're tempted to do something other than pursue your goal you're going to have to argue with yourself. If you aren't completely convinced that what you're doing is truly worthwhile, temptation is eventually going to win.