Best Practices for Speeding Up Your Web Site — part of an excellent series of articles on the Yahoo Developer Network.
They mainly focus on the front end, as that is where you’ll get most bang for your buck. They make frequent reference to YSlow, a Firebug extension that looks extremely useful.
Some things were familiar to me (and most are already standard practice for my colleagues), many weren’t (e.g. avoid @import for CSS), and some I know of but haven’t been convinced of before (e.g. CSS sprites). I was quite intrigued by this example of CSS sprites in use on Google. (When using CSS sprites, remember to consider their accessibility to screen readers, and don’t use if you want them to print.)