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.