Low-level format your hard-drive several times. (easy with SCSI because the host adapter usually has this ability built-in, but should be easy enough on IDE drives as well).
For total destruction, a very powerful pulsed magnetic field will totally scramble the HDD to the point where it may even become unwritable ever again.
Physical destruction of the HDD means opening the case and smashing the heads/platters with an axe/sledgehammer/crowbar/etc. Buyer will have to acquire and install new HDD.
Removing HDD and keeping as souvenir, backup/paperweight also a very good option. A spare HDD is always handy.