Remove all applications except for the Apple apps, and of course remove all of your data, reset Safari, delete all user account except for the Admin. Make a new user account (I called mine "new owner"), reboot the machine and log into the new account that your just made, delete your old user account. Go to Applications, Utilities, Disk Utilities, Click on your Primary HD, Select the Erase Tab, then look in the lower right side of the windows for Erase Free Space. Click the Security Option button first and choose 3-pass, then click the Erase Free Space. DO NOT CLICK THE ERASE BUTTON. Depending on your HD size it will take from 5-10 hours, but your are as safe as you can be that your information is gone, short of replacing the drive.