I took a tip from BradB and cut the bar into many pieces. I take the
clay, knead it out until its about 1/4" thick, and then cut the bar into as many silver dollar-sized pieces as I can get (6-8 usually). Then I just use the coin-sized piece until it picks up visible dirt/grime, then flip it over. Once I've used both sides I toss it out. Works great and I get good mileage out of one bar. And add to that the fact that you don't have to worry about kneading the clay (and the risks inherent in it), and its a great method.