From Leo Babauta:
Have a powerful reason — when things get difficult, “because it sounds nice” or “to look good” aren’t going to cut it.
Start tiny, with a simple but unbreakable promise to yourself to do one small thing every single day.
Watch your urges, and learn not to act on childish whims.
Listen to your self-rationalizations, and don’t believe their lying ways.
Enjoy the habit, or you won’t stay with it longer than a week’s worth of sunrises.