Overwrite all partitions, the master boot record, and data:

dd if=/dev/zero of=/dev/sda bs=4M

This will overwrite everything with zeros (0).

Overwrite (more securely) all partitions, the master boot record, and data:

dd if=/dev/urandom of=/dev/sda bs=4M

This will overwrite everything with random data.

Progress check:

# to see the progress of dd enter:
kill -USR1 $(pidof dd)

Repeat to check again.

To wipe out your MBR:

dd if=/dev/zero of=/dev/hda bs=446 count=1

Wipe out MBR -AND- partition table:

dd if=/dev/zero of=/dev/hda bs=512 count=1