I'm retiring a really old PC and still have a drive in it containing sensitive data. (Sensitive to me anyway, not like the drives that the government loses with millions of social security numbers...) I'd like to give the PC away to a nearby charity, but want to ensure that the data is gone. It turns out that the best way to clean a drive is simply to overwrite it with zeros, ones, or randomness.
The easiest way is zeros, just:
dd if=/dev/zero of=/dev/sda1
Or whatever your device is. If you want ones, it's a bit trickier since there isn't a /dev/one device:
cat /dev/zero | tr '\000' '\001' | dd of=/dev/sda1
Supposedly, you are supposed to write and rewrite over the drive or some hacker can figure out how to recover the data. This is not true. In fact, there has been a competition out for some time to reward any company who can perform this feat. It has never been taken.