Make sure sensitive deleted files cannot easily be found again

[Roquefort]

Step one: Throw personal belongings into water body.
Step two: Hunt & forage for food, and live in a cave.

Seriously though, Just wipe your free space with something (not as in literally wiping your hard drive with rubbing alcohol) and you should be good.

[epsilon88]

I'd like to also point out that these methods may not be as effective on SSDs when compared with a regular magnetic disk because the former employs wear leveling. When you overwrite a file, it gets mapped to a different set of flash cells on the disk. So your data might still be there.