Well, I say this every time this topic comes up, but I think it bears repeating.

Tape a piece of paper over the drawing surface.

Ever since I've done so with mine (both at home and work), I've found that I'm actually able to free-sketch into Flash and Painter, to the point where I have been doing animations from pencil roughs to inked and colored work entirely in the computer.

Gives you more friction on the pad, and feels more like actually drawing with a pencil (rather than a wax crayon on glass).