Great question! Welcome to the forums.
There are a couple options.
You can insert your restore disc and reboot holding down the C button until you see the gray apple. Follow the instructions and look for the extra option to securely erase your drive. "Erase free space". 7-pass should do it, but it'll take all night.
There's also an app you can buy, but personally I don't think its worth it.
Edit: Just to let you know, the app is called SuperScrubber.