A reasonably brief article that gives some information about how to clean a drive is at PC World: Answer Line: Wipe Your Drive Clean of All Its Sensitive Data.

The article includes links to some programs, both free and "real," that should be reliable, even though the article is rather old (2003). There are large numbers of "freeware" programs easily found with a search; but it's hard to verify that such programs actually do what they say.

The comment in the article that a 13GB Hard Drive (do they still make anything that tiny) took "12 hours" to wipe suggests what might be required to wipe "modern" 500 GB Drives. The new drives are a bit faster than the old ones, but you can probably time the process with a calendar if you use the most secure forms of wiping.

(Incidetally, the links all still seem to work, but I didn't check whether the prices on the unfree versions may have changed.)

Short of running the drive through an "industrial scrap grinder" that reduces the largest bits to bunny pellet size, physical destruction of the drive is not really secure against recovery of much of the data. A "wipe" with a known good program is better than anything you can do to it with the tools you're likely to have at home - including your stash of C4 or the PETN you brought home from WWI or II.

John