- Backup regularly - I backup and reinstall every month. Leopard comes with Time Machine. Use it! Yes, it can also be used with drive partitions but that won't help if your HD fails.
- Have a Linux live CD available - It's only a couple minutes download and burning; if your computer fails, you can use the live CD to get your data back.
- A couple spare flash drives - Most people have one or two flash drives (or jump drives, or whatever you want to call them). Keep one or two in hand as emergency drives. Pop in the Linux live CD, insert your flash drive in, and copy some data from your hard disk to the flash drive. If the data is very sensitive, such as source code or other types of stealable information, delete it off the hard drive too.
Here's a tip: don't save important data within the OS-given folders, such as "Documents", "Pictures", etc. (Home not included), but rather place them in separate directories under your home folder, which gives you more of a chance to access it via the live CD.