The seemingly best solution I've heard of is this:
Put a velcro ring around the ignition key, and on the disk lock. Cut both into donuts to allow the key to be inserted with the velcro ring (the loop or rug side)
Cut a circle tab out of the hook side.
Put the circle tab on the disk lock, covering where the key goes. To unlock it to put in the key, you have to remove the velcro tab. After you remove it -- stick it on your ignition covering the ignition keyway.
When you come back to your motorcycle, you have to remove the circle tab from the ignition (reminding you to unlock the disk lock, and put the velcro circle tab back on it.)
easy, simple, cheap, and mistakeproof.