This is a followup on my post: Digital Cameras: What size memory card should you buy?
My husband is of the philosophy that one should never delete a photo. He says that he never knows what he will learn in the future that will allow him to take a picture that is not very good and turn it into a great one. Of course, he loves to fool around in PhotoShop and other digital imaging software.
Still, for the rest of us, the idea of never deleting an image works great until we have a chance to back up the memory card. While my husband and I are on vacation we have enough memory with us so that we do not have to reuse a card. If my laptop is with us, the images get backed up to my computer and then my husband cuts a DVD of the pictures. He also downloads the pictures to a memory card back up device.
He uses a unit made by Wolverine. We never go on vacation without it, whether I am taking my laptop or not. As far as my husband is concerned, one can never have too many copies of their pictures until they are safely stored at home.