Unless you have some desire to hit things with a hammer
NAME shred – delete a file securely, first overwriting it to hide its contents
SYNOPSIS
shred [OPTIONS] FILE […]
DESCRIPTION
Overwrite the specified FILE(s) repeatedly, in order to make it harder for even very expensive hardware probing to recover the data.
Boot up Knoppix from a memory stick
shred -vfz -n 10 /dev/{Drive}
Overwrites the entire disc with random data 10 times then writes the entire disc with Zero’s.
http://www.howtoforge.com/how-to-securely-destroy-wipe-data-on-hard-drives-with-shred
Normally about 3 overwrites is enough – technically 1 should do as beyond that recovery is into the theoretical. This was the US DOD standard but they swapped to crush/industrial shredder probably as people didn’t believe that a software wipe would work.
A word of caution this method has NO UNDO BUTTON