I am sure that most people who do polymer clay know this trick but I include it in case there are beginners that haven't stumbled on it yet. It is sort of an extension of the previous Tidy Clay post, I find that the handiest and most effective way to clay up polymer clay bits is with a sheet of conditioned scrap clay. You just press the sheet in to the stay bits and instead of throwing away a cleaning cloth you just run the mess through the paste machine and it is ready to go again. Of course that the scrap is always good for bead cores and a dozen other used, so with polymer clay there really isn't thing as truly scrap clay unless it has hardened beyond redemption.
If your clay is almost beyond redemption you can take a look at this old post.