If you do not run your mac 24/7, the odds are the built-in unix system maintenance scripts* will not run. This is a problem if you are running a server, as the files may become huge (up to hundreds of megs). The easy way to solve this problem is to download MacJanitor (link below).

MacJanitor

You can also run the scripts from the command line using the command 'sudo periodic daily' then repeat, replacing "daily" with "weekly" then "monthly."

*here's what the scripts normally do:
Daily:
Removes scratch and junk files
Cleans up NFS turds
Backs up NetInfo database.
Removes system messages older than 21 days
Checks subsystem
Rotates log files (system.log)
Cleans web server log files

Weekly:
Rebuilds locate, whatis databases
Rotates log files (ftp.log lookupd.log lpr.log mail.log netinfo.log hwmond.log)

Monthly:
Does login accounting
Rotates login logs.

Daily is usually run at 3:15AM, weekly at 4AM on Saturdays, monthly at 5AM on the 1st.