deleting files and reformatting your hard disk won't completely do the trick

two graduate students at MIT's Laboratory for Computer Science bought 158 used hard disks

they were able to discover 5,000 credit card numbers, personal and corporate financial records, medical records and personal e-mails.

60% of the hard drives had been reformatted, and about 45% of the drives had no files on them

What can you do? Get a disk-wiping program

Computerworld features editor Valerie Potter vouches for the free Darik's Boot and Nuke

A similar program that has gotten good reviews is Eraser.

If you've got a Mac, you can use Apple's built-in Disk Utility

A superb resource for figuring out how to reset cell phone data is put together by ReCellular, which buys, recycles and refurbishes wireless devices. Its cell phone data eraser site gives detailed instructions on how to erase data from many different makes and models of cell phones.