I use Aperture, so I use the vault feature to back up. I have one 500GB drive connected via USB and a 1TB drive connected via FireWire. I have a vault on each of those drives.
I backup whenever Aperture tells me something's not backed up, whether it's an adjustment or a batch of images.
I also plan on backing up jpg's of my best images on dvd, as jpg's have more chance of surviving time than RAW files. Haven't gotten around to it yet though.
I'm currently in a bit of a pickle though. My photo library grew from nothing to 70GB in just over two months, and my computer only has just over 4GB of space left. Since I'm averaging over 30GB of photographs a month, I may have to move my library to my 1TB drive (still working on a better plan). This will probably screw up my backup plan.
The only thing I can suggest to improve your backup plan is to sign up on one of those online backup websites. That way if something happens to all your onsite backups at once there will still be an online copy.
Edit: I agree, NikoDoby, we've gone through quite a few drive failures as well, can't trust any hard drive. Too many moving parts. Solid state drives look promising, but they're small and expensive. The best backups are the ones done automatically, like shooting on film. The film is your backup if you're working on the scans. Stuff the slides in one of those fireproof vaults and your set ;-). Same with camcorders that use DV tapes. Import to your computer, and keep the tapes as a backup.