What always gives me the most motivation is looking at old pictures of myself when I was a normal weight. I have started pulling them out once per week to get a firm visual image of myself at my ideal weight. It is working so far.
Also, I look at old clothing that I've kept, even though I'm sizes too big to wear them anymore. I haven't kept too much, but I like to take them out and try them on - sometimes it makes me laugh, sometimes I feel pretty blue about it, but it normally helps provide a kick-in-the-arse for me.