I'm not quite 'out there in the real world' but whenever I'm limited on money and whatnot, instead of trying to go ahead and add it all up by the exact price I'll round it up to the nearest dollar. Sometimes I'll even round it up two dollars. If it's something like '$25.98', I'll figure $27.00. That always helps me not spend as much.
Making a list and strictly following the list helps as well. I wouldn't try to spend money on things unessential, or things that you really don't have the money for.