I remembered reading a research paper years ago that suggests overwriting a drive only once makes the data unrecoverable. However I just found the article again and it seems to have gathered a lot of criticism. So despite what I was going to point out, it seems a multiple-overwriting strategy is justified.
On the other hand, one of the other linked research papers talks about another commonly suggested technique, magnets. The conclusion is that while floppy disks are easy to erase with magnets or degaussers, for other media such as DAT tapes and modern hard drives, no known degausser is powerful enough to effectively erase them. In fact, hard drives contain powerful rare-earth magnets right in there next to the platters which are used to move the head. From the article:
even the most powerful commercial AC degausser cannot generate the recommended 7,500 Oe needed for full erasure of a typical DAT tape currently used for data backups