Rapid Proofing Material

Hi,

What do you guys use as a proofing material when you want to test out
quickly whether a part will come out like you want. I'm tempted to
use wax since it's nice and soft, and you could remelt all the chips
when you're done and mold it into the shape of more or less any stock
you wanted. What worries me is that it might simply melt like a
candle and gum up the cutting tool/leadscrews/etc etc.

So far I've been using some soft pieces of wood I found around my
house, but it's not been without trouble. The fine particles of dust
got into my leadscrews after a few cuts (Okay, it was my fault for
being impatient and blowing the dust as the mill was cutting because
I wanted to see the part :P) and the mill started making horrible
sounds and losing lots of steps. I had to take the whole thing apart,
clean it with acetone, re-oil and recalibrate it. That said it moves
REALLY smoothly now, the Y-axis purring like a cat. :)

So what do you use for rapid proofing?