I saw your post about destroying 3.5" diskettes. Best method I've found is to tear them apart, then cut up the magnetic film inside. The plastic is just a shell, the magnetic film is where the data resides. There are sometimes companies which offer this, and occasionally (at least around here) community shred events that may have a sufficient power to destroy them (home use shredders won't because of the metal ring). I also have heard the advice of dumping them all in a bin of water, adding salt and letting the magnetic section corrode.
I work for a State level tax agency. In the past we would do the bin full of water after drilling holes for hard drives, but eventually hired a company to come out and destroy them and our floppies for us. We pay about 1 USD per item destroyed, but they also destroy hundreds or thousands at a time, making it worth their while to come out. For small batches the cost may be higher.
Hope this helps =]