audipower,
I think a lot of people do this anyway, but if you're not going to use Accum's "disposable clay" technique, here's a tip or two on how to get the most out of a piece of clay:
First, don't use a whole bar at once; 1/4 to 1/3 or a bar is nice size that's not too small, nor too large (for my meaty paws, anyway). Flatten the clay into a rough rectangular shape, and clay a panel with the lube of your choice as you normally would. Now, if the surface of the clay is dirty, don't just roll it up and knead it around, then flatten it again. Instead, dab any excess lube off the flat piece of clay with a clean MF or other towel (I usually just tap the clay on my shirt to get any moisture off of it), then fold it like a book with the dirty side to the middle. Then, gently flatten the "book" with your thumbs and fingers into a rectangle again. By always folding the dirt to the middle, you expose two clean sides of clay. The dirt becomes stratified in the clay with the contaminants trapped between layers of clean clay.
I can usually do several cars with a piece of clay (all depending on the condition of the car, of course) before discarding it. If I feel that a piece is nearing the end of its lifespan, it gets relegated to glass, then wheels, then the bottoms of the side skirts, underside of the bumper, etc. (the grimy places). After that, I pitch it.
YMMV, but this technique has worked well for me.
Tort