Alison has posted a guide to a technique that can help your site resist higher loads - content caching:

Caching your database-driven website pages has a plethora of benefits, not the least of which being improved speed and reduced server loads. This article will explain how to set up a simple caching system, and will also address when and where caching might not be appropriate.

She shares her "six lines of glory" that cache the resulting HTML page (after all data has been pulled from the database and rendered) into a cached file with output buffering. On other side of things, there's another script that, based on the page requested, checks to see if a cached file exists. She also modifies it slightly to include a timestamp so things aren't just cached forever.

There's also a recommendation in the comments of the post to use the APC caching extension that's already built. Of course, this can only really be used if you have access to install it on the remote machine (not an option on things like shared hosting).