It’s important to format your cards two different ways at two different times. #1) I generally format my cards before leaving my apartment for a gig. This gives me the opportunity to make sure I’ve downloaded all of the shots on the card before deleting them and makes sure I don’t run out of space while shooting. #2) I periodically do a low-level format of all of my cards on my desktop computer immediately followed by formatting the card in my camera. This wards off write errors on the card by making sure the memory is completely clean.