Apple is constantly providing developers with test builds of the next OS update so they can ensure compatibility with their programs, drivers, and system services. When an update is nearing its public release date, Apple may increase the number of developer releases and some may be tagged with "no known problems" or similar phrasing. When this happens we may see the update released in the next few weeks if not in a few days, but before you decide to install it be sure you have your system prepared and have considered options for installing.
Apple designs OS updates to be installed over practically every setup, so do not worry yourself too much with being prepared; however, there are steps you can take to ensure things go as smooth as possible.
Always back up your system before updating it. The best practice is to perform a full and restorable backup by using Time Machine or a cloning system (SuperDuper, Carbon Copy Cloner) and then testing the backup to be sure you can access it in the event of an update failure.
To test your Time Machine backup, boot to the OS X installation DVD, select your language, and choose the option to restore from backup that is in the Utilities menu. In the restorable backup list, you should see the most current backup you made. To test a bootable drive clone, boot to it either by selecting the clone in the Startup Disk system preferences and rebooting, or by holding the Option key at start-up and selecting the drive from the boot menu.
Once you have confirmed the backups are healthy and have booted back to your main hard drive, unplug the drive from your system (if you can--some people use internal drives for Time Machine) or unmount it at the very least (just drag it to the trash) so the system will not interact with it during the update. Then proceed with the update.
Clear up current issues
If you are having major problems with your current OS installation, try addressing them first. While OS updates can be the solution to many problems, if you are having major stability problems (i.e., random crashes, odd noises, inability to authenticate) then be sure to address them before applying the update.
Run general maintenance
At the very least, run some general maintenance on the system before updating. For the most part you can do this by booting into Safe Mode (which runs a few maintenance routines at start-up) and then running a permissions fix using Disk Utility. In addition, you can also clear caches and other temporary files using programs like OnyX, Snow Leopard Cache Cleaner, IceClean, Yasu, and Cocktail.
For more in-depth guidelines for running general maintenance, see our article "MacFixIt 'General Maintenance' recommendations."
Unplug peripheral devices
If you have external USB or Firewire devices, unplug them from your system before updating. While it is rare that peripheral devices interfere with installations and updates, it can happen especially upon the first reboot as the system reconfigures drivers and boot caches. Once you have installed the update and have completed the first boot, then plug in your devices again.
This is the most common method of updating, and will download the minimum number of files needed for your system and current software setup. It is the fastest and easiest method, but will keep a large number of the unchanged files on your system.
Standalone Delta update
As with other versions of OS X, Apple will provide the update as a standalone installer. This may be a larger download than what is available via Software Update because it includes update files for all computer models and software setups. It will be available at Apple's support downloads page, and we will also provide a link to the delta updater when it is released.
Using this update allows you to take extra recommended precautionary steps during the installation, such as booting into Safe Mode and installing when unplugged from the network to avoid any interruptions.
Standalone Combo Update
Similar to the Delta updater, you will have the option for the full Combo update. This update will contain the full set of files that have been updated since the OS's initial release (ie, 10.5.0, or 10.6.0). Using it to install the update will ensure all updated files are replaced, even if they have not been changed since the previous intermediate or step release. Using the combo updater is a fairly standard troubleshooting step that can help fix various OS problems, and is a good way to keep your OS installation as fresh as possible.
As with the delta update, we recommend you install this when booted to Safe Mode and after running standard preparatory maintenance routines such as permissions fixes.
We are always looking forward to the next version of OS X and will do our best to cover any issues that crop up with the update, so be sure to check back for updates. While we do not expect there to be any major problems with the update, there may be unforeseen bugs with some drivers and devices, so it may be wise to allow others to test the update first, especially if you rely on the stability of your computer and the peripheral components you use.
Update: This article was originally written for the OS X 10.6.3 update, but has been modified to apply to all upcoming OS X releases.