Instead of fighting the easy way, why not orient yourself so that the easy way is the disciplined way? Note that when you want something and have no doubt you can get it, you simply do what you need to do to get it. If you're thirsty and a glass of water is within reach, you just grab it and drink. There are two conditions for this easy achievement:
- I want something.
- I have no doubt I can get it.
The more attainable you think your goal is, the less bothered you are by the effort necessary for its achievement. When you think it is difficult to get something, you might rationalize that you actually don't want it or you might make yourself forget that you want it. Thus the second trick to maintaining discipline is convincing yourself that what you want is attainable. Say, "This is easy," or at least, "This is straightforward".
On a related note, you must believe you deserve the goal of the discipline. Otherwise, you'll just sabotage your discipline by making its achievement evil or seem harder than it is. If you notice yourself mad at people who have made it while you haven't, realize that your thought of "he doesn't deserve it" will subconsciously drive "I don't deserve it". You'll secretly give yourself reasons to fail so you don't end up like that jerk who succeeded. Instead, use their glory as inspiration for yours. Everyone deserves the happiness of a disciplined life.
Fourthly, measure your progress. When it comes to conditioning, our minds don't operate much differently than Skinner's rats. Positive feedback acts as positive reinforcement for the habit we are trying to instill. A popular measurement strategy is the streak. Count how many days in a row you've kept up an activity that is part of your discipline, e.g. how many days you went to bed at a specified time.
You can also measure your progress by regularly rating the quality of the product your discipline is creating. Even if improvement seems imperceptible one day to the next, record yourself. For example, record yourself practicing a musical instrument. For writing or visual arts, keep your practice pieces. For motivating physical exercise, keep track of how energetic or tired you feel at various points during the day. Over the course of a few days or weeks, you'll notice a correlation with the amount of practice and measured benefits and be naturally encouraged to continue.
If you keep these four ideas in mind--remembering what you want, knowing it's attainable, believing you deserve it, and measuring your progress--you will maintain a momentum that reduces the need for sustained application of will-power and the resulting fatigue.