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Firstly, you want to make sure that store your cards in archival quality bags. That sounds like it`s going to expensive and difficult, but it`s not really. You just have to make sure that the bags you use are made from plastic in the "poly" family. That is, bags made from polyethylene, polypropolene etc. They usually aren`t expensive at all, and you`ll be able to put your cards into them knowing that they will be safe from deterioration for a long time.
A couple of companies that sell bags suitable for cards are:
Bags Unlimited - they will send a free catalogue and you can place your order online.
Dandl Products - They sell boxes of bags suitable for greeting cards. The price I saw on here was $11 for a box of 1000!