Haven't had to do it recently, but if I remember correctly a good low level format (done by BIOS) changes every bit to zero effectively cleaning the drive. Some information may still be stored in hidden sectors, but it won't be your every day skeletons. The last time I did that it took all night on an old 512mb HD.</p>
Writing all the data in one direction doesn't work. The relative strength of the bit field can still tell you what the ones and zeros were.