If you have less than 512 MB of RAM, you should definitely upgrade your RAM. I recommend at least 1 GB, and more if you plan on running many applications at once or some very large applications. The general rule to follow is buy as much RAM as you can afford.

Apple is continuously optimizing Mac OS X. By having the latest version, you will have the most optimized version. Not only that, but many of the new features make mundane tasks easier, meaning you will spend less time doing the boring stuff, and more time having fun.

You know those cool windows that come out of the title bar instead of the old modal alerts (for example, if you choose Print from the file menu)? These are called sheets, and you can manually speed up their drawing time. Simply issue the following command in the terminal:
defaults write NSGlobalDomain NSWindowResizeTime .1
This sets the display time to 0.1 seconds instead of the 0.2. You can choose any number you want, including larger numbers to slow down the sheet speed.

Current Mac laptops and consumer desktops can accept hard drives that spin up to 7200 RPM, while the current Pro desktops can accept drives that spin up to 10,000 RPM. For the most part, the faster drive you use to boot OS X from, the faster your entire experience will be. Other hard drive factors to watch for include cache size, seek time, and cool new technologies such as vertically bit storage.

If you have more than one hard drive in your system, one of the best things you can do is RAID them. There are many levels of RAID, with the three most common methods being RAID 0, 1, and 0+1. Here are the differences in these types:

RAID 0 - Also known as the non-RAID (because of the lack of redundancy), this type reads and writes to all drives in the array at once. This makes it the fastest RAID available, but also the most unreliable. If one drive goes down, they all go down. Can be called stripping as well.

RAID 1 - This type of RAID is mirroring, which means that every time you write to your hard drive, an exact copy is written to the other one. This is the greatest level of protection, but the least speed increase. In fact, write speeds will not be affected at all, while read speeds will be marginally faster, as the first drive to find the data is the one who reads it.

RAID 0+1 - Using this method requires at least 4 hard drives. In this case, two are stripped while also being mirrored. This provides great backup capabilities, and shows very good speed increases. I would recommend this if you have enough drives.

Keeping the number of files down will reduce directory size, which will help when your hard drive needs to search for files. Use the sub tips below to help keep the drive clean.

i) Run maintenance scripts
These scripts are run late at night if your computer is kept on (and not sleeping), but otherwise will not run automatically. If you don't leave your Mac on 24/7, use Maintidget to run them manually.

ii) Eliminate extra languages
There are thousands of extra language files on a standard OS X installation that most of us will never use. To get rid of them, use Monolingual.

iii) Delete printer drivers
By default, OS X install hundreds of megabytes worth of printer drivers. You can either delete the ones you don't need by hand (they are located in /Library/Printers). If you don't feel safe doing that, you can use either Printer Setup Repair or Print Center Repair to delete those files.

The default genie minimizing effect is very slow. If you have Mac OS X 10.2 or later, there is a setting in the Dock System Preferences to change it to scale. If you have not yet upgraded to Jaguar or later, you can use TinkerTool to change it.

There are many fonts included with Mac OS X, some of which are only used for foreign languages. Panther includes a utility called FontBook which allows you to easily disable unwanted fonts. Just make sure to leave the Helvetica fonts enabled, as disabling them will make your system unstable.

Starting with Jaguar, Apple added something called Quartz Extreme which speeds up 2D graphics dramatically. If your video card does not support Quartz Extreme, but your computer can accept a card that does, upgrade for enhanced performance.

Printer Setup in Mac OS X 10.4 (and likely also Print Center in older versions of the OS) keeps track of all the print jobs you have completed. Sometimes this list either gets corrupted or very long. In either case, your print jobs may be taking longer than normal. If this happens, follow these steps to clear that list and restore your print times to an acceptable pace:

1. Open Printer Setup Utility (In the Utilities folder)
2. Double click your printer
3. Click on the completed tab
4. Use the Jobs menu to select Clear Completed Jobs List

Even if you don't log out and back in (or restart) too often, it's still a good idea to keep your login items trimmed. These programs not only force your login times to be longer, but also use up valuable resources (sometimes completely in the background) while your use your computer. Use System Preferences -> Accounts -> Login Items to see what you currently have set up, and decide if you can live without them auto-starting.

The current way Apple implements desktop icons is kind of silly. Each icon is actually a full screen sized window, that takes up quite a bit of RAM. This translates into the more icon you have on your desktop, the less RAM you have available for other applications. In fact, if you have over a couple hundred icons on your desktop, your system could slow to a halt!

When Apple introduced that they would be switching to Intel, most developers thought this would be a nightmare. Luckily, Apple made the transition fairly easy by making tools available to allow for compiling of programs for both PowerPC and Intel Macs. If you have a new Intel Mac, check all of your third party applications for the latest version to see if there is an Intel or Universal version available. If there is, this version will undoubtedly be faster than its PowerPC counterpart.

If you are running Mac OS X 10.4 (Tiger), then Spotlight is automatically enabled for each hard drive connected to your machine (internal and external). If you don't see a need for searching your external drive via Spotlight, you can disable the indexing, which will speed up various operations (especially when connecting the drive). To disable a drive, goto System Preferences -> Spotlight -> Privacy, and drag your external drive to the list. You can also do this for your internal hard drives, but that removes the benefits of having fast searching capabilities.

It's unclear how much of a benefit this tip will provide, but at least it will help clean up your computing life. Using the tool Service Scrubber you can disable any of the services listed under the Application Menu > Services. Everything listed here is present because some application has registered these services. I imagine that it not only takes some CPU power to maintain this list, but also enable/disable certain items on the fly depending on your context. Therefore, eliminating entries from this menu must provide some benefit (if nothing else, it will make the Services menu usable again).