Physically destroying them (hammer, drilling, etc) is the most secure, but I don't like to recommend that simply because it's a waste of good hardware. Me, I boot to a live Linux disk/usb and use GPartEd to delete the partitions and write a new partition table. Quick, simple, easy-peasy. That's good enough as far as I'm concerned. You'd need forensic tools and mad skilz to get anything off them afterwards.
If you're really paranoid, a command line program called 'dd' can be used to write zeros to the drive. It meets all but the most stringent DoD specs. There used to be a boot disk called DBAN that did the same thing that used a rudimentary GUI, but I haven't used in in forever. If you've got a local IT geek, they can do it for you. It's not a highly technical process for someone with basic skills. Or if you want to pay me big bucks to drive to Jasper...
