Since we're talking about files and folders that may not be obvious, consider using one of the following tools to help with the task. Disk Inventory X (free/donationware), WhatSize (US$13; free trial), GrandPerspective (free/donationware) and OmniDiskSweeper (free; enhanced version US$15) are all designed to look at your hard drive as a whole (rather than browsing through individual folders) in order to give you a clean and unbiased picture of your disk usage.
Once you've found the large files on your hard drive, you can choose to delete them, move them to an external drive, store them inside a compressed disk image or archive them as a .zip file.
10. Increase RAM
The final tip for this article is probably the most well-worn piece of advice for any computer user wanting to boost performance: Add more RAM. Any computer will perform faster and better with additional RAM. RAM provides working memory space for the operating system and running applications.
Leopard can allocate RAM very effectively and will swap data from RAM to the hard drive if need be, but having more RAM to work with will certainly increase performance. In particular, Intel Macs that rely on integrated graphics, where the system RAM does double duty for both regular computing and video memory, will benefit from more RAM.