Hi,
degaussing your HD is the way to go... Thats what we did in the Gov when we had classified information. However, be warned, degaussers are very expensive.
Putting it in the microwave, as previously suggested, would be very dangerous (due to a possible leak of EM waves rebouncing off the metal components of the HD), plus, you could simply damage it (no more pizza pops for you!)
For floppies, we would break open the floppy, (after having thouroughly erased the content), take the medium , sandwich it between two pieces of paper and put it in a cross-shredder.
For cds, there are utilities and hardware to remove the first layer of a cd, removing the data it once had but again, its pricy.

hope this helps,
banshee