I recognized that my Apache 1.3 consumes a lot of memory during different tasks (eg. running phpDocumentor multiple times allocates about 80 MB of RAM for every instance). You can gracefully restart your web server to free the allocated memory, let's say every five minutes. Graceful means that no process is killed while it is running.
I put this into a cronjob (for root) with crontab -e:
0-59/5 * * * * /usr/sbin/apachectl graceful
Note: This is probably only suitable for development web servers, since it will disrupt access to the server during the graceful restart...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051115043110429