Thanks for the responses, folks.  I didnt want to risk ruining a model by reusing silly putty but I wasnt fond of the idea of replacing it for every model.

Craig

I reuse it till it gets too full of paint. Beware, though, it doesn't play well with all paints. Works great with all acrylics I have tried, but don't use it with Tamiya spray cans. The solvent or maybe the propellant in the spray cans liquifies it into a mess that's real hard to remove! Otherwise, it makes a fantastic masking material.

You might like this stuff as well, it's amazing just to play with let alone masking. Bit expensive but should last for years. There's reviews and Utube vids out there about it.

Panzer Putty.

http://www.mxpression.com/miscellaneous.html

Mick.

Likewise---just knead it 'clean' and use again. I've got two alternating 'eggs' I've been using for several years, still going strong.

I reuse mine, you might want to toss it when it gets really loaded with paint but so far I've had no issues.