UPDATE: This is unlikley to be as effective on newer SSD based hard drives due to their wear leveling technology.
To wipe a disk (eg sda) use the following (multiple times if paranoid):
# dd if=/dev/urandom of=/dev/sda ; syncOr a quicker version which is not as secure but should be fine unless you have pissed off GCHQ or similar (in which case its probably already to late anyway):
# dd if=/dev/zero of=/dev/sda ; syncTo determine current progress find the PID of the dd process then pass it a command using kill:
# ps -A | grep dd
# kill -USR1 <pid>