I second Secure Erase. Note that this is not a utility or a product, it's an SATA command. It tells the drive to empty itself. A good hard drive will take hours to execute this command as it writes random garbage over every location several times. An SSD will execute it quickly and, most important, update its list of used blocks to show that all the blocks are free.
If you don't use this, you end up with an SSD badly in need of TRIMming, as it thinks that actually unused blocks are unavailable for writing.
Most boot CD environments will have a utility to issue the Secure Erase command.