I have something like 30 domain names that I own, so I’m always looking for a way to save a few bucks on registrations.  GoDaddy.com always has some special offer for a percentage off, or reduced prices.  I used to keep all of their emails so that I could go back through them when I needed to buy something. I just came across this site that displays all of the currently active and recently expired coupons. Now I can unsubscribe to their newsletter and keep it from filling up my inbox.
Also, I stumbled on the ‘bulk pricing‘ on godaddy’s site.  When renewing 6+ domain names at a time, you can get a discount as well. Over 20 domains and the prices are about the same as with discount codes.