Today is Leopard day. Apple's sixth major upgrade to the OS X operating system goes on sale today in the United States at 6 p.m. I'll be heading to the Apple Store in Atlantic City at 6 to grab my copy of the cat and will install it on my MacBook Pro shortly thereafter.
Before diving into such a massive upgrade as 10.5.0 there are some precautions that you should heed.
Step 1. Make a full, bootable clone of your current hard drive to an external USB or Firewire HDD. Not just the Documents folder or your Home folder either, clone the entire drive. I use SuperDuper (US$28) for this task because it's simple and powerful, the Smart Update feature alone is worth the price of admission. Those preferring a free option can use Mike Bombich's excellent Carbon Copy Cloner 3.
After cloning your drive, test it by booting from it, launching your email client and a couple of your critical documents. If everything seems ok, move on to step 2.
Step 2. Verify your drive's health. Boot from the DVD that came with your machine and run Disk Utility from the "Utilities" menu. Run Repair Disk on your drive until it comes up with a clean bill of health. If it comes up with lots of errors, run it again. If you don't have the original DVD, you can run it from Applications/Utilities folder, but the DVD is preferred.
Step 3. Remove all third party preference panes, login items and menu bar items. This goes without saying, but it's best to uninstall anything that modifies the OS (especially those visual tweaks) and didn't come from Apple.
Step 4. Instead of installing Leopard "over top," consider doing an Archive and Install of the new OS. This process automatically moves existing system files to a folder named Previous System, then installs Mac OS X again–from scratch. To save time you can opt to preserve your user and network settings before installing which imports existing users, Home directories, and network settings and skips the Setup Assistant after installation.
For further reading I suggest David Morgensterns' Planning for a Leopard migration, David Pogue's
What are you doing to prepare for Leopard?