PrePressProduction.com / tips / clean system

Format the target drive with low-level formatting. Many sources, including Apple, report that low-level formatting of the drive may remove imperfections in the drive that a normal erase/format will not affect.
Perform a clean installation of the operating system. Ensure that you are installing a version of the operating system that supports your computer model; installing newer systems on older models may result in slow performance and system-level errors.
After restart, follow the on-screen instructions to set up the basic settings for your computer.
Install any other 3rd party software that you need. You may need to restart after this step.
Open your Extensions Manager and turn off all unnecessary extensions and control panels. This is a very important step. Unnecessary extensions and control panels will slow down your system. They may also conflict with each other (this only applies to 3rd party extensions... Apple's default system extensions will not conflict with each other).
Open your Memory control panel and turn off virtual memory. Always use real RAM rather than VM (this applies to pre-OS-X systems only, of course, since OS X uses VM by default).
Set your Keyboard control panel's "key repeat rate" to "fast", and "delay until repeat" to "short". This is invaluable for fast navigation and scrolling.
Set your mouse speed to the fastest setting, to reduce hand/wrist movement as much as possible.

Maintenance

Always run Norton Utilities at least once a month. Specifically, Disk Doctor and Speed Disk. You should reboot off an external disk in order to diagnose and optimize your drive properly.
It's a good idea to run anti-virus software on your Mac. Norton Anti-virus and Virex are recommended.
Rebuild your desktop regularly (I suggest once a week to once a month). You do this by restarting and holding down the command and option keys, until you see a dialog box that says "do you want to rebuild your desktop?". The desktop database keeps track of where your files are, and can sometimes get corrupted. Advanced users should delete the Desktop DB and DF files and let the Finder create a fresh new desktop file: I recommend free software that does it automatically for you, such as Total Desktop Rebuild, Trash Desktop, or Techtool.
Zap your PRAM at least once a month, especially if you have plugged in any new hardware, including external drives, RAM, and cards. To do this, hold down command-option-P-R after restarting (after the chime). Hold down the keys until you hear at least three restart chimes, and then release. You may need to reset some of your system preferences, in particular the Date and Time settings.
Do not indiscriminately install 3rd party software... especially shareware/freeware software such as screen savers.
Always check new fonts to see if they are damaged before installing them. Suggested software: FontAgent, Fontdoctor, Adobe Type Manager Deluxe, and Font Reserve.

Use British spelling in your searches