How to permanently clean a HDD

Basically I'm selling my old PC and I want to say how to erase all the stuff in it without risk of getting some of the information formerly stored stolen by someone (such as medical and personal informations).

I know that the safest way to deal with that is a hole in it, but I think that the buyer would not be so happy about that option.

deal erase all hdd information medical old personal privacy risk selling stolen