You can use the Erase Free Space feature while booted into the system itself.
First, delete your user files: I recommend that you create a new, admin user, log in as that user, and then delete your previous user(s) completely using the Accounts preference panel. (Choose to "Delete immediately", not save as an image). Also uninstall any applications that you aren't passing on the disks or licences for.
Then, using Disk Utility, click on your HD on the left side, click the Erase tab, and use Erase Free Space to write over the now-empty space on your drive. One pass of zeros is usually quite sufficient (unless you have military secrets).
Matt