Eraser
Most people reading this probably already know that if you delete files, or move to another location/encrypted container/etc. Remember that the original isn’t really deleted, only the filesystem’s record of the file being there is deleted in order to allow future use of the space. (Which is why unerase programs work.)
It’s also the reason that if you want to securely erase a file, you should use a tool that doesn’t just erase the file, but also shreds it. In other words, it overwrites the data with random noise, (multiple times if you want) so it can never be recovered.
This makes unerase programs useless, so if someone steals your unencrypted drive or laptop, they can’t go snooping for deleted files. (And if you’re paranoid and think the NSA or similar adversary with a very large budget is after you, shredding multiple times prevents any lifting of your data via magnetic force microscopy.)
To accomplish this, use a something such as Eraser:
No, not that kind of eraser…. This kind of eraser. You can wipe your recycle bin, unused space, or specific files or filetypes, both on-demand or scheduled. (If you have a lot of space to wipe, it can be slow, so you would want to schedule it to happen during off-hours.)
It’s open-source, easy to use, and seems to be well-trusted by those in the security-conscious community.