I still prefer to just not carry money at all
You sound like every woman I've ever gone out with!
Linked on the Instructables page, I found a neat game along the same topic. The idea is that you get 10 [US] coins (3Q, 2D, 1N, 4P) and a whole dollar to pay for an amount of $.XX. It's pretty good, actually--there're a few levels of "difficulty" coded in. If you're not bored to tears by my description, try it here
edit: A better description would be: mental gymnastics.