I just used yslow to enhance the performance of my site. I managed to bring it from lowest grade F to grade B - nice!
This firefox plugin can be run on any website and shows 22 indicators how your site performs (= basically what is happening when the site loads in your browser). Some easy things you can score on: 1. put the JS at the bottom of your documents, 2. make fewer HTTP requests (putting more scripts into one file, because the browser loads only 2 files at the same time!) and 3. "pack" code files (eliminating white spaces - there are tools for this, see packer and jscompress (my bookmarks).
Chapter 3 of "building findable websites" (see reading list) describes how this works and much more.
Yslow for faster websites!