Brandon Savage has posted the next article in his "Scaling Up" series, a look at reducing the amount of "drag" your application makes through its processing path and some tips to help increase its "lift" out of some common problems.
The intuitive will note that many if not most of these suggestions are performance enhancements, not scaling techniques. Why then are they in an series about scaling? Scaling is about more than just adding hardware. It's also about making sure your system runs better. You can add lots and lots of hardware but you will someday be unable to compensate for bad queries and poor optimization.
Some of his suggestions include taking care of any sort of errors or notices (anything that could slow the script down by writing to a log), defining virtual hosts instead of making excessive use of .htaccess files and installing caching software to maximize code and information reuse.