If you use online backups in conjunction with a local hard drive, you can kill two birds (multiple backup media and offsite backups) with one stone. Thats also simpler than rotating disks from your home to a friends house or a safe deposit box. But whatever technique you use, be sure you have a backup somewhere safe from the dangers that could affect your Mac.
Walk through your restoration plan
The final -- and perhaps most crucial -- component of a heavy-duty backup system is a restoration plan. In other words, think through in advance precisely what you will do to restore your data, whether the problem is a single missing file or a dead hard disk. When you've just lost important data and are stressing over a deadline, you don't want to have to figure out how to restore files -- or worse, discover that your backup software wasn't doing its job properly.
Don't merely read the instructions for restoring your data; try it. In fact, performing test restores should be one of those tasks, like fire drills, that you perform every so often just to reinforce the steps. Adam Engst of TidBITS suggests putting a reminder on your calendar every Friday the 13th: International Verify Your Backups Day. Take a moment to make sure you can restore older versions of a few key files, and confirm that you can start your Mac from your bootable duplicate. That way, if and when a real problem occurs, you wont have to panic; youll be secure in the knowledge that your data is safe and know exactly how to recover it.