Where i live decent cards arent cheap so i try use my decks as long as possible before i open a new one so for most of the time ive got a "poop deck" anyway so if i need someone to sign a card ill just take another card from my older deck to replace it, also make gimmicks out of them
I've made some artwork for my walls with a bunch of cards that were not fit for my performances anymore...not only is it a good way to preserve cards that you can no longer use, but every once in awhile it is a good way to mentally relax and get creative too.
I either use them to practice the mercury card fold or I use them in effects that I have printed things on the cards like lip / kiss prints or black finger prints and start giving them out and letting people keep them at the conclusion of the effect. Or...I simply have just tossed some in the trash and cracked open a fresh pack. It's not that hard to do if you are only getting them from Costco or Sam's and not paying much more than a buck a dec.