I have been shooting on P2 for about a year now. As mentioned, one of the most reliable solutions would probably be LTO/DLT for long-term storage. You must also take into account your budget as a LTO system will end up running you around $4K to $5K.
What I have been doing is buying CalDigit VR Firewire drives. Single enclosure external unit with 2 drives in it. I set it up as a mirrored RAID and offload my raw P2 cards directly to it. This makes 2 copies of the raw footage, one set on each drive. If a drive fails down the road, I will always have the backup.
The key to P2 is file management. I have been shooting my stills digitally since ’99, so I was used to backing up files.
The most important thing is to make sure you are using an offload program like P2 Genie or HD Logger. DO NOT DO DRAG AND DROP! This may work for a while, but eventually you will lose files since it randomly writes files and does not do a file-for-file verification of the card. Hope this helps!
John
HPX-2000 w/AVC board
P2 Mobile with AVC board
MBP 10.5