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1. Install More Memory/RAM
Whatever OS you have Mountain Lion or no ML RAM always, well 99% of the chance will improve your computers speed. The reason I say 99% is because sometimes even if you add more RAM like in Windows XP 32-bit more than 4 GB will be useless because it wouldn`t register (XP only goes up to 4 GB [32-bit])
2. "Mess" with the Dock
Even though the dock is visually appealing it does take up your system`s memory and I`m not saying to delete all your apps on your dock but just disable the genie effect for example as well as turn off animating windows when opening your apps. (When the icon jumps up and down)
3. Disable Desktop Animation
If you change your desktop picture every 5 seconds or so that takes up memory that you will actually use with something you wouldn`t even be able to see if you have a full screen app. Just uncheck the box "change picture every...."