I started doing this a little while ago, and it has been a pretty big help for some of the various things I've gotten.
If you are anything like me, you got to be in the 100lb club by enjoying some serious junk on a regular basis. I got here, most likely, because of my late night snacking and thrice weekly fast food dinners.
I won't lie, I still want the fast food, pizza, Chinese, (even Ben and Jerry's!) what have you, that is super high in calories, fat, sodium, etc. HOWEVER, what I've started doing is when that craving is pretty strong, I'll take the money I would have spent on my fast food meal (usually $5 or $10) and stick it in a jar, then go make something healthy. When I want new fitness gear (work out clothes, dvds, gadgets, home equipment), I have a stash of cash to go to!
Though it might be nice to use that as my reward cash for things like mani/pedis when I hit bench marks too.
something funny: my fast food jar is a tostitos salsa con queso jar.
I don't know if this will be helpful to anyone but I thought I'd share.