Most domain name registrars such as GoDaddy, offer a variety of promo codes. If you have many domain names to renew, it is smart to take few minutes and search Google for a “godaddy renewal coupon,” which will let you find currently active discount codes.
However, keep in mind you can usually use just one promo code per purchase and many of them apply to selected extensions only. The very obvious idea is to renew all of your .com domains, then separate all of your .org etc.
If your purchase is for a significant amount (for me, renewal time means an expense of few hundred), you should always call to your domain registrar’s support and ask what they can offer. I use this trick with GoDaddy and in many cases they have their own secret promo-codes with a fixed percentage for discounts, which gives me more savings than a checkout discount I will find online.
Savings can be significant, never rely on the auto renewal option.