Restoring worn-out deck of cards.
You need deck sleeves.

They should be available at your local game store, or from any number of online suppliers.  They are available in various sizes, so be sure to bring a sample card along while shopping (or measure it precisely before hand).

Online you can find lower quality sleeves for around 1 cent apiece in bulk.  Higher quality sleeves may cost upwards of 10 cents/sleeve.  The higher quality will give you a sleeve that shuffles much easier and can optionally have an opaque back.  (An opaque back is desirable if some of your cards are marked or bent due to heavy use).

Ultra-Pro and Fantasy Flight are two high quality well known manufacturers to ask for if you want somewhere to start.