These are questions I'm asked so often, and I'm sure anyone who doesn't ask it still wonders at some point! The idea of something malfunctioning and resulting in lost images is something I take very seriously, and I've taken extreme care to make sure it won't ever happen.
First, any camera I use MUST have 2 card slots, meaning that any photo I take, I'm automatically recording to two cards. It means I automatically have a backup right from the get-go.
Second, I always shoot weddings with 2 cameras. Always. The main reason I do this is so I don't have to change lenses during fast-paced parts of the day, but it also means that if a camera body stops working for whatever reason, I have a backup on hand!
Third, when it comes to backing up images, I like to make sure I have lots of backups. I automatically back all the images up to two separate hard drives as well as online, so they are aways in three places minimum.