Ever hear of a bill jar?
The idea is that you pick a denomination- let's say the 5-dollar-bill. Every time you get a $5 bill, you put it in to a jar. Some folks then make savings deposits once they hit $50 or so.
I like this idea, but there are two problems with it.
One: You have to have the iron will not to spend that money. If you can handle knowing you have $45 in a piggy bank in your bedroom without splurging on a nice dinner, a movie or something fun...then it might be a way to go. Then again, you probably don't have to trick yourself to save money either.
Two: Who uses cash anymore? That's the biggest drawback I see to this plan. So unless you do the cash-in-envelopes system to pay bills or just use a lot of cash- this one probably won't save you much.