Anyway, if you need to keep a whole deck but want to have cards signed, you can get a one-way force deck and just force the duplicate then replace it after every trick...
Already been suggested earlier on.
The blank card idea is nice, but unneccessary. If you're worried about going through too many decks, simply don't give the card away at the end.
If you are interested in a Blank Card Ambitious Card Routine, look up Greg Wilson's Point Blank on Pyrotechnic Pateboards. He uses a completely blank deck and the spectator's signature ends up merged with his own.