OK, the subject line is a bit tongue and cheek, but with the number of people who claim updates "kill" their systems I had to catch those who are about to update before they feel it has killed their system.
BACKUP YOUR DATA
I mean it, do it at least twice to two separate mediums before you install anything. That includes updates, patches, firmware, etc...
No data is invulnerable.
Ensure that all the periperals and applications either have been tested with the update, or at least one requires it by either asking here on the Apple Support Communities what software and hardware variations have known to work, and what does not among your collection.
Make sure your hard disk won't be over 85% full after the update, this is an arbitrary number where Mac OS X seems to slow down at.
Make sure nothing is causing permanent freezes that don't let you force quit applications with command-option-escape, or don't let you switch to another open application. If there is something permanently freezing a program short of force powering off the machine from the power button or power source, and you don't get the shut down dialog when you hit the power button, there may be something wrong with the hardware or software installed which will NOT be solved by an update. Note, the force quit command won't save your data either, so saving work from applications frequently is always recommended in event something like that should happen.
When you update your operating system, shut down the computer first and detach everything but the display and wired keyboard and mouse on a desktop, or everything that's attached to a notebook except power. You don't want access to your backup to be restricted after the update, which has sometimes been known to happen. In those events installing the older operating system, and combo update and using the Migration Assistant on the backup tends to recover the system.
All other suggestions are voodoo in their origins, and really should be ignored unless you wish to troubleshoot a backed up system. But backing up is the only thing that will save your system from certain death.
And did I mention backup your data?
Please don't forget.