To get the discount, just mozy on over by clicking here, go to the signup page, and enter "OCTOBER" in the "Promotional Code" box when you check out.
By the way - I've been using the service for over 6 months and still think it's great. I've had to restore some of my data a couple of times and it worked flawlessly.
Not convinced yet? There is a freebie plan that will let you back up 2 Gigabytes of data.