I use a 1 shoot, 1 card system. This does not mean that i will actually use only 1 card, but i never have 2 different shoots on the same card... This way it is easy for my to keep track of shoots and card usage!
Ok, so you've been out shooting, you have all your images on your card and you transfer them to your computer and back-up system (read here for more info about what I do). I manage my cards in such a way I know which one's are "used" and which ones are "ready" (watch this), so it is easy for me to track. Usually I transfer my images to my computer/back-up as soon as possible, but this is not always the case. So when faced with a wallet with lots of "used" cards, this is what I do.
- Put card into camera and see what is on it, since I batch my cards (1 shoot = 1 card), I can see which shoot it is right away, and will check on my computer and back-up system that it is there.
- Assuming #1 is cool, FORMAT the card IN THE CAMERA WITH WILL BE USED IN!!!
- Repeat for all "used" cards.
Now, why format and not delete? Well, there are some particular reasons why. Now, just to get the record straight, formatting and deleting essentially do the same thing. They do not remove the image information from the card, they just remove the index to it, so neither your computer or camera can see it... the reverse of this is how rescuing lost images works.
I think deleting is a bad habitat to get into, I never want to manually delete from a card, as one day, i will delete something I didn't want to. I know you can rescue delete items, but from my experience, this is not a failsafe option, more of a last ditch attempt!
Formatting, however, goes one step further by also removing and replacing all file structure information too. It is VERY important to format the card in the camera you intend to use it with, or at the least, the same brand of camera (even though this is no guarantee). Because of my 1 shoot, 1 card rule, I can safely format a card when I know which shoot it is and have it all backed up!
Some cameras have a "low level format" option for SD cards, this works better than a quick format, which is just a glorified delete all. So make sure you use that! :)
Remember, always format your cards as soon as you have everything backed up, this way you are ready for action when it comes! :)