I'll only buy myself toys on weeks when I've stuck to my diet.
It's pretty clear that I can't control my spending. I'm not terribly worried about it, because my overhead is low, my income is (reasonably) high, and currently all my roommates are paying their rent and bills on time, and generally, I don't go for high-pricetag items (though a stack of books adds up to an expensive thing!) and I've only bounced one check in my life. But I do shop all the time, I do buy a lot of books, and I do occasionally blow $200 on some obsession or another, even when I've been telling myself I'm not going to spend money.
It's even harder to control my eating, particularly this time of year. It's challenging because I'm a cook, and I particularly love cooking dessert. So, after new years, this'll be my promise--if I've broken my diet (beyond the three bites of a dessert or tiny microcheats Stage 2 South Beach allows), no new dungeons and dragons books, electronic doo-dads, or whatever.
I think I can stick to that. I'll either cut down on my sugar and starch intake, or cut down on the amount of money I spend on impulse purchases. Either way I win--assuming I have the willpower to keep this promise, but it's pretty generous (sure, have dessert, just risk not being able to get that Speak'n'Read when it turns up at the Salvation Army).
Starting January 1, anyway. Because I wouldn't have given up that Katz's Deli cheesecake shake for a very large sum of money :)