Where I work we use a 4kg mallet and sell it without the harddrive - at auction. No chance of it being resurected.
If you have ever used your credit card or stored passwords for internet banking then this is really theonly way to be sure the this information cannot be resurected. DoD (US requirement is write three different values to each bit and repeat this 7 times.
You can use a product called Boot and Nuke which I believe is also reasonably secure. Personally I like the way work does it.