My theory is that playing cards work like a thermostat, as the cards get warm they warp because one side of the card expands while the other side remains rigid. Humidity makes this even worse.
I have decks that I have used for years that still look pretty new and handle pretty well.
Lyle and I think alike it seems!
1. Keep your hands clean: Like has been said this can kill a deck of cards.
2. Keep your hands dry: Unfortunately, some people have sweaty hands, I do not. My hands are always bone dry to the point that when I hand a deck out to be shuffled it feels clumpy almost immediately. I assume that this is because my hands are generally dryer than others. That leads me to my next point.
3. Don't let other people touch your cards: Regular people don't have the care that we do as magicians. They also aren't as sensitive to the feeling of a deck.
4. Don't carry the deck in your pocket: When you carry the deck in your pocket it introduces all sorts of junk into the deck, sweat, dust, lint ect. The only exception to this is to carry the deck in a Porper card clip or better. This protects the cards even more than the box.
5. Don't drop the deck: If you drop the deck on a relatively clean surface you still may damage the cards. Dust, scratches and bumps all affect how the deck handles. Dropping the deck is a sure way to kill a deck. Also, make sure the surface you practice on is spotless. It is not wet or dirty, just dry and pristine.
6. Certain sleights can kill a deck: Cards are resilient, at least a good deck is. That said, there are tons of sleights that bend and warp cards. Things as simple as an improper bridge or cascade can warp a deck. When I first learned how to back palm large portions of cards this killed my decks.
7. Crappy decks clump quickly: At Comic Con I bought a few decks of cards with a "magician's" finish. This was sold as better than an air cushion but in this instance the cards clumped almost immediately. Bad, cheap, decks don't fan or ribbon spread right. Cambric finish, or air cushion finish have been the best for me.
8. Don't just use one deck: Give your cards a rest sometimes. Use a deck for a day then switch it the next day. When your cards get a second to dry out and "relax" they last longer in my experience.
I hope that helps!