This is probably going to sound ridiculous but just make up your mind and do it. That is what has worked for me, and I only have my personal frame of reference for advice
I decided I was going to get healthy and decided that the time was NOW. Every time there is a question about "should I eat this?" I have an answer ... "nope, I made up my mind".
I think goals help too whether they be your big end goal or many smaller little goals. Fixate on something, whether it be a new BMI, or a significant number on the scale, or little rewards. Example: buy a pair of pants 1 size smaller then fit you now and work on staying on track until they fit. Wear them and rejoice... then go buy the next smaller pair (and yes, I am currently in the smaller jeans I had bought. They fit now, and now I am waiting to get into the awesome green boutique pants I bought 1 size too small. They are so cute. I can't wait!)