The Apple support site has some tips on running Time Machine and how to perform a full restore of your system if something goes wrong.
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You should also run Disk Utility (type “disk utility” into Spotlight), select Macintosh HD, and click Verify.
This will verify the integrity of your hard drive and make sure there are no corrupt files that will get in the way of the upgrade procedure.
In some cases, application developers only need to release a new version of the installer — but in other cases, installed software will simply refuse to run under OS X Lion until an updated version is released.
Before you install Mac OS X 10.7 Lion, make sure you run through this list of preparatory precautions: Roaring Apps is a community-powered site that has been testing application compatibility for OS X Lion over the last few months using the beta and final, golden master builds.
Apple has loosed the Lion update unto the world and into the hands of feverish, excited Mac fans.
Before you fork over your for an upgrade license, however, be warned that Apple’s OS X updates tend to break some applications — and in the case of Lion, which deprecates some older Apple machines, you even need to make sure your hardware is supported before you run the installation.
Click the Apple icon in the top left of your screen, Software Update, then follow the prompts.
In the unlikely case that something goes wrong during the installation procedure, or if you discover that a vital application doesn’t work under the new version of OS X, you’ll wish you had a backup.
Fortunately, Time Machine makes backing up and restoring your full system incredibly easy — as long as you have an OS X installation DVD, anyway.