How to completely remove deleted files from your Android device
Deleting files usually just opens up their storage space for overwriting, meaning part or all of them can still be recovered long after you thought they were gone. Whether you're working with sensitive info or you want to be sure your device is safe before you sell it on Craig's List, you may want to ensure that deleted files are not recoverable. ShreDroid is a free app that sends those files to oblivion--learn how to use it in this blog post by Rob Lightner.
When is a deleted file really gone from your device? Only when the last bit it used for storage has been overwritten. This is generally not an issue for most users, but if you're an international superspy or (somewhat more likely) you're selling or trading in your old device, it's important to scrub it in order to prevent outsiders from learning more than you want them to. ShreDroid is a free app that overwrites free space, though it has caused problems with some Motorola devices (see below). Here's how to clean your Android machine:
Open it up to set the (rather simple) options. Select External and/or Internal Storage, then whether or not to run as a background process. This is the only way to automate ShreDroid, but it can be a memory hog, so if you've got a reliable system to prod you to use it regularly, manual shredding is probably best.
That's it! It takes time and processing power, but your data will be long gone when it's all over.
Motorola users should follow this FAQ from the developer to see when ShreDroid will be ready for their devices.