Its different for everyone, but I find the biggest motivating factor for me is looking at the progress I've made and saying to myself "You did that, you worked for it, now you're gonna trade it for... this?" I keep a food diary on Livestrong and then I have a pocket calendar that I keep up with my "days primal" in. If I have a day where I slip up I still enter whatever I ate into my food diary and I write on my calendar, NOT PRIMAL. In a way I'm competing with myself against how many days I can go completely primal. The food diary is helpful because I know if I eat something that doesn't fit in the diet I'm going to have to enter it in on my daily consumption, it keeps me honest. It helps stop the "Oh just one M&M wont hurt..." Just one turns into 8 then comes the peanut butter sandwich and all hope is lost! But I know if I have to enter in that one M&M I'm less likely to grab it.
"It is not what you are that holds you back, it is what you think you're not."