Good guide from IBM...
With Apache, the general idea is to eliminate extra checks the Web server must do, such as processing the .htaccess file. You must also tune the Multi-Processing Module you're using to balance the system resources used with the availability of idle workers for incoming requests. The best thing you can do for PHP is to install an opcode cache. Keeping your eye on a few resource settings also ensures that scripts don't hog resources and make the system slow for everyone else.
http://www.ibm.com/developerworks/linux/library/l-tune-lamp-2.html?ca=dgr-lnxw01LAMPTuningP2
