Handle with care. Maybe this goes without saying, but it's possible to be rough with your cards. We've probably all seen people shuffle cards so crudely that we visibly grimace! If your hands are tense, and you grip the cards too tightly, or bend them excessively while shuffling, you can cause unnecessary damage. Of course it's equally possible (and perhaps even likely!) that you know how to handle cards carefully, but your friends or family don't! As a result, if you give them your beloved deck to shuffle, they might be very rough with your cards, and that could simply be because they have never learned the proper techniques for shuffling or handling a deck. Be gracious, of course - but you might want to offer to be the designated shuffler or dealer for the card game.
Spring the cards. Not only is springing cards an impressive visual flourish, but it can also play a very practical purpose of actually helping your cards stay in shape. A card that is being used positively is a happy card. Cards that just sit there and are never used can run the risk of being warped, just like being laid up in bed for weeks will make you stiff and out of shape. In contrast, a good workout with the help of a spring or shuffle can assist in making the playing cards keep shape, by clearing out all the cobwebs or dust (literally!), air them out, and give them some valuable restorative exercise.
Be bi-directional. If you do springs and riffle shuffles, make sure that you don't just do them the same way all the time, e.g. only face up or only face down. Spring and shuffle them in both directions from time to time, otherwise the cards will always be under pressure to bend the same way. This constant pressure from the same direction will affect the fibers of the cards, and can cause them to be permanently bowed in the long run.
Don't drop them! This may sound obvious, but dropping your cards is asking for trouble. The first casualty of a dropped deck will usually be the corners of the cards, which risk becoming bent in the process. If you want to fast-track your deck to becoming one of those dog-eared items, throwing your cards around is definitely going to speed up that process. Furthermore, any time your cards spend on the floor means that they're likely to come into contact with dirt that has been tracked in on the carpet, linoleum, concrete, or grass, or wherever you happen to be using your deck of cards. Getting sand in between your cards is especially something to avoid, because this will cause extra friction in the wrong places, and will speed up the wearing process when you shuffle the cards.
Practice above carpet. If you are a cardist attempting a range of new moves of fancy aerial moves, you don't want to be doing this above a filthy ground or a hard wooden floor or concrete. You are going to drop cards. Yes, I know that this conflicts with a previous suggestion where I said that you shouldn't drop the cards. But dropping cards occasionally is part of the cost of progression in cardistry or magic. Every good cardist will drop cards in the course of learning and attempting new and challenging moves. If you never drop cards, then you obviously aren't challenging yourself or pushing yourself to new heights. And if you know that you are going to drop cards, then it makes sense to have them fall on a surface that is going to be as friendly to them as possible. A clean and soft carpet is best. A hard and dirty wooden floor, or a muddy puddle outdoors most definitely isn't!
Do nothing. There's an old saying that "time is a great healer", and it can apply to playing cards as well. Of course, if your two year old nephew has chewed off the corner of your favorite Ace of Spades, no amount of time is going to make that corner grow back - not even your best Torn and Restored magic routine! But sometimes when a deck of playing cards is starting to feel clumpy or spread unevenly, it just needs a break. Just like a car can overheat, sometimes a deck that has been handled for a long period of time simply needs a breather. So put it back in the box, and maybe in a card clip if you have one, store it in a cool and dry place, and give it a chance to dry out and get back to normal. You may find that in an hour, a day, or a week, the cards handle better again. Just like a holiday can do a stressed person a world of good, an overworked deck can benefit from having an occasional break, so give it an well-deserved and therapeutic rest from time to time!
No rubber bands, please! We've all seen it: a deck of playing cards, secured tightly with a rubber band. Don't do it. Why not? Well first of all, over time that rubber band is going to become brittle and break. Worse, when you add some heat it's going to melt, and you'll have bits of rubber actually stuck to your cards. Yuck! Furthermore, there's a real risk that the rubber band will damage the cards at the top and bottom of the deck, because it puts pressure at those points. A rubber band does help keep your deck together, but it offers zero protection for the cards themselves - and we can do better than that!
No pants pocket, please! Sure, it's nice and warm in there, and it seems to be a safe spot to put your cards. And sometimes you'll have no option but to put a deck inside your pocket. But think about it: a deck that's pressed tightly against your body is going to warm up. It may feel romantic, but when romance is in the air, things can start getting sweaty and hot, and that's a sure-fire way to make your deck start warping. Pants pockets also tend to put pressure on the deck when you walk around or even when you sit, and this can quickly cause damage to the tuck box, or cause the whole deck to bend. If you do need to carry your deck inside an item of clothing, try putting it in a jacket pocket instead. And if you really have to resort to using a pants pocket, try putting your deck inside a card clip or some other deck case or protector first.
Use the tuck box. There's a reason why playing cards usually come in a tuck box. Tuck boxes are certainly important for marketing and branding, and especially in the case of more classy decks that feature embossing and foil accents on the tuck box, they make an immediate statement of style. But they also serve a very important and practical function in protecting your cards. If you leave your cards out in the open, they are vulnerable to moisture, and will also attract dust - and perhaps even some spider-webs or other nasties that really don't belong in your deck! So use the tuck box, and look after it! You can always patch it up with duct tape if you really need to! Remember that your tuck box is your first line of defence against playing card enemies like dust, dirt, and even against sunlight and moisture.
Store your decks flat. You'll find that opinions on this subject do vary. But it can make a difference whether or not a deck of cards is stored in an upright position or flat. When stored flat, gravity is on your side, pressing the cards flat against each other in a natural way. When stored upright or at an angle, there is a greater possibility that your playing cards will warp over time. Whether or not this is an issue for you can depend a lot on your environmental conditions, like the temperature and humidity of the place where your cards are being stored, but you can help combat those other playing card enemies by storing your deck in a flat position.
Avoid humidity. Sometimes you really can't do anything about the environmental conditions where your deck is stored. But humidity is particularly known to have quite an impact on a deck of playing cards, so if there are ways to store your deck in a cool and dry place, away from sunlight and humidity, and with a relatively stable temperature, definitely that's the preferred option. Wait a moment, does that sound like your fridge?! I have heard of people who swear that putting a deck in a fridge overnight is the best way to improve the condition of a warped deck, and that it's also an unorthodox fix for cards that have that undesirable "click". I haven't tried the fridge treatment myself, because there can be a lot of moisture lurking there too, so it sounds like a bad idea to me, and I can't speak from experience. But if you're really desperate, have exhausted all other options, and are willing to experiment with a particularly rebellious deck, you may want to give that a shot as a last resort! But generally speaking, try to avoid storing your deck in a high moisture area that encourages your cards to curl and warp. Fluctuating humidity is even worse, because cards will expand and shrink, and quickly become damaged. A cool, dry, well-ventilated area is always the best. If you live in a climate with high humidity you might want to put your decks in the same room as your household humidifier if you have one.
Avoid sunlight. Sunlight has a tendency to bleach, and if something is left in direct sunlight for extended periods of time, it will inevitably get damaged. You can't buy sunscreen for cards, but you can keep them away from the sun, by ensuring your cards are stored safely in the tuck case when they're not being used. This also applies when your playing cards are inside the tuck case - don't leave it on the ledge of your bedroom window or on the dashboard of your car, where the tuck case is going to sit for hours in the full sun.
Avoid heat. Direct sunlight also invites another enemy of playing cards: heat. And of course there are other sources of heat besides the sun, and adding heat is another sure-fire way to damage your playing cards. This isn't rocket science, obviously, but I wouldn't want to be standing underneath the burning flames of rocket engines, would you? Similarly, it's hardly ideal for your playing cards to be exposed to significant amounts of heat. Heat can accelerate chemical reactions, and changes in temperature will cause things to expand and contract. Inevitably, this will lead to problems like warping, which you really want to avoid. The solution is simple: if you can, try to keep your cards at a constant temperature, and don't store your deck right beside your fireplace or on the window-sill.