The New Earth Online has a quick look at one easy method for speeding up your site in a few different ways - caching pages and information with things like Cache_Lite and APC.
As your site traffic grows it takes longer and longer to generate a dynamic page from sending multiple queries to a database. One possible solution to limit queries is to cache the result of each query that is needed, or to have a complete full page cache for your site.
They look at the two ways I mentioned - the Cache_Lite PEAR package and the APC extension (that will soon be included by default in the stable PHP releases). Bits of code are provided for each showing how to get them set up and get them working inside of your application.