Consider keeping old software installers
Apple doesn't keep them, so you may want to
Our number one recommendation for mitigating the negative effects of a problematic system update, or any other software revision for that matter, is the creation of a startup drive clone with a utility like SuperDuper! or Carbon Copy Cloner directly prior to applying said update. When things go wrong, the clone can be quickly copied back to your original startup drive, or you can simply startup from the clone and get back to work immediately.
If you are for some reason loathe to clone (lack of a properly-sized external drive, an odd phobia of clones, etc.), an alternative is to keep old copies of old update installers handy. Apple does not generally maintain old versions of various applications -- like iTunes -- nor some system update revisions (delta and combo Mac OS X updaters are notable exceptions). As such, keeping older copies of system updates on hand can sometimes allow you to revert your system to a state prior to the update without needing to reinstall Mac OS X or restore from a clone.
There's one hurdle to jump when reverting to earlier versions of applications or system components, however. Many installers will not allow you to overwrite newer versions of the items they contain. This can be easily overcome with the aid of Pacifist, which can force installation of packages. Just open the old-version package with Pacifist, click on the name of the package in the displayed list, then click the Install button in the upper-left corner and follow the on-screen instructions.
Feedback? Late-breakers@macfixit.com.
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