How do you light the fire? You have to decide you want X more than Y. Life is really a bunch of trade-offs. How do you motivate yourself to lose weight or to exercise (there's always temptation, you're tired, or any number of excuses) - you have to decide you want it bad enough. Think about things that you've accomplished in your life (college, running a race, whatever), how did you motivate yourself to do it? Are you internally or externally motivated? Do you do better if someone other than yourself is holding you accountable? Think about patterns in your life and apply them to FI motivation.
Someone else mentioned conscious living - I think that is the key until it becomes second nature to you. If you find that you're making wrong choices (or choices that prevent you from FI), then do something that will constantly remind you of your goals. Maybe it's putting positive visual reminders in obvious places (dressing mirror, car dash, fridge, wallet) until it becomes part of your natural being. Another way to think about something that you want is that you can have it, just maybe not now.
In the beginning it may feel forced - you have to repeatedly make conscious decisions. But then, over time is just becomes part of your habit and you lose the desire for impulse purchases, unnecessary eating out, etc. - it becomes "who you are". You no longer have angst over purchases, you don't even give purchases a second thought because you're no longer an impulse purchaser. It takes a while to change habits, so be patient and don't beat yourself up if you stray - just get right back to it.