We have never lost an image from a wedding due to the following backup workflow for each of our events:
During the event, we shoot on two cards at the same time. This instantly creates a backup of every image in the camera. Afterwards, we separate the primary cards from the backup cards, should anything happen to one set. We then back up the images to a local server set up in a RAID 1 configuration, as well as copy them to an offsite hard drive. At any given point before delivery of the images, there are two copies of the files in separate locations.