Making data on hard drives unrecoverable

How does one make the data on hard drives completely unrecoverable?  I am selling off my old computer but I do not want the information on the drive falling in the wrong hands.  Am I correct that just formatting the drive is not sufficient and most data recovery programs can easily recover data from formatted drives?

They can recover some of the data. It depends on whether it's been over written or not.

Most folks use Darick's Bopot and Nuke (DBAN) to do multiple passes of wiping the hard drive.