In the Mac OS X GUI, Under the Finder Menu go to Preferences [Command ,] then go to the Advanced Tab and select Empty Trash Securely. Now there is a trick to this as I discovered if you just click on the trash can and request to empty files it will do a standard deletion nothing secure about it!
There are two ways to delete the files securely.
1. Open the Trash Can in the top Right of the Trash it have a button Empty Securely click it and it will empty the trash using srm from the gui, however it will not use a DoD secure delete profile by default.
Note: The -s option overrides the -m option, if both are present. If neither is specified, the 35-pass Gutmann algorithm is used.
2. From MacOSXHints;
What if you want to remove a file but not use the Trash? The new feature was introduced in Panther & is the command/usr/bin/srm. There are two other options that secure empty trash doesn’t use:
overwrite the file with 7 US DoD compliant passes (0xF6, 0x00,
0xFF, random, 0x00, 0xFF, random)
after overwriting, zero blocks used by file
So, srm -mz [filename] will do a DoD compliant erase and zero the data.
Hope this helps!