How to Erase Hard Drives
There are many ways to erase your hard drive, rm can do it for you. But what if removing them is not enough, what if you need to make sure the data can not be recovered?
 Using DD
dd is a handy tool used for making copys. It can also be used to quite securely wipe harddrives:
for (( i = 0;i<10;i++ )); do dd if=/dev/urandom of=/dev/hda done dd if=/dev/zero of=/dev/hda
This will overwrite your entire hard drive (in this case seen as hda by Linux) with random information ten times, then with zeroes once at the end.
This tip will take literally days or even weeks to run on an average machine with a decent sized drive.
Huge thanks to John Cairns, Ritter and EnIgMa.