Mostly the weight loss is a reward in itself. But I also get a lot of pleasure out of wearing clothes I haven't worn in a while, or realizing old ones no longer fit me. I also have a 35 dollar gift certificate to Urban Outfitters which I will use once I reach my goal weight.
On a smaller scale, though I don't have a "cheat day", I do allow myself two pieces of pizza and a glass of milk Friday nights as a reward for a week well done. If I behave well the rest of the week, I find it actually has little to no impact on my weight loss, and whenever I feel like binging, I can tell myself to wait until Friday, instead of telling myself no forever.
Good luck!