1. Build a routine. Set a specific, repeating time when you will do the thing in your calendar. Keep that slot clear, ALWAYS. Never let something interrupt this task.
2. Learn to say no. If someone wants you to do something else during this time slot, say no, and tell them why.
3. Never break the routine. Breaking it once makes it MUCH easier to break the next scheduled time. If you do break it, feel bad about it and get back on the horse IMMEDIATELY.
4. Use the power of accountability to reinforce the routine. If you can find someone who will hold you accountable, do it. Someone who does the routine with you, or a coach who will call you out if you make excuses.
5. One thing at a time. Don't build some huge routine of 15 tasks at once. Ease into it one task at a time.
6. Don't overload yourself. Leave time in your schedule for play. If it gets to be too much, decide CONSCIOUSLY which one you will drop permanently (and not right before the schedule to do it).
Edit: I'll also say this: Overcoming adversity builds discipline. A tough life that forces you to fight for what you want builds this naturally. An easy, carefree life doesn't make you tough. Seek out tough things to toughen yourself up.