One of the ways to ensure your memory cards last longer is to format them each time you insert a card into your camera. Formatting the card erases all the remaining stored information & sets it up to work properly with that camera. It's a fairly simple procedure & should be easily found in your camera's manual. If you can't find or don't have the manual do a internet search for an e-manual or call the company & see if you can get a replacement. Ebay is another good place to find missing manuals.
Since it becomes an expensive proposition to store all the images I capture on sd cards, I move them onto my computer after (almost) every time I go out to take photographs. At most I will only store a few days worth of images & remove them with in a week. (Partly because I am so excited to see what I ended up with!) I always remove them from the card after a shoot for a specific customer. If I move them from the card after every session then I am not digging in my gear wondering what I can & can't delete. I keep all the cards that I've used in that session in a different place in the bag than the clear ones. After moving them to the computer(s) & sorting them, I will move them onto an external hard drive, upload them to my internet storage site, & also onto a disk. Having them stored in a variety of places creates a safety system to ensure they don't disappear if something crashes or other unforeseen occurrence. I know there are other photographers out there who store them on disk or other medium then they send them to another photographer or family member for storage at a different location (a different town is best, a different state would be awsome). This is a great idea to protect yourself from losing files in fires, tornadoes, floods, and any other natural or human disaster that might occur.
Once I have my images stored I can feel free to reformat the memory card & clean up all the fragments that might stick around when just deleting the images, like defraging a computer.
Another excellent tip I received is DO NOT shoot an entire session on one card. Don't go out & try to buy the biggest card you can so you can cram everything on one card. Buy multiple smaller cards. The 4 GB cards work well for me & I can buy several for the same price. The reasons are fourfold. 1. You don't want to spend a ton of money on one card that will eventually go out on you (or that you might loose). 2. If you store all of a clients images on one card & it gets lost, damaged or decides it has an unrecoverable error, you have the other cards and you still have part of the images. Can you imaging telling the bride you lost all her images? How about the new mom who had newborn pictures? Or even images from your own family reunion or kid's birthday? 3. If you fill up a large card it will take most computers & card readers FOREVER to transfer them across. And 4. It gives some people too much temptation to let the images sit there on the card & nothing gets done with them, especially if they rarely take photos. Like film in the refrigerator, "Hummm.....I wonder what's on here. I haven't pulled pictures off in quite some time. Oh look! There's my son's 1st birthday & kindergarten graduation & high school graduation!" Ok, so that probably wouldn't happen, but you get my point.