DBAN's "Boot and Nuke" disk eraser is a convenient way to completely erase a hard drive or other storage medium.
You create a diskette, boot from the diskette, and then select the storage device you want to erase. There are a number of settings that can be used depending on how paranoid you are. If you're erasing truly sensitive information, it does not hurt to be very paranoid.
This program is open source and free to use. There are a number of other programs that do this type of work, but this looks to be one of the more convenient ones out there.