So a large magnet left on the drive for a while might be the best way to go shy of destroying the drive then?
I wouldn't bet on it. There's too much metal in the case to count on the magnetic field wiping out the bits on the platters. Physical destruction is the only way to be sure. The software suggested above to write multiple ones and zeros over your data is good enough unless the CIA thinks you are important enough to rewire your hard disk or look at the platters with an electron microscope.
Maybe. That's one I can't really answer. I don't know. But I'd be surprised if HD's are suceptable to any but the largest external magnetic sources.