They have pretty much stopped dicounts for renewals. There are sometimes some $1.00 off your order and $2.00 off .COM renewals but that seems to be it.
If you have a lot of domains (I am not sure what the minimum is), you can get an account rep and get most domain renewals for $6.95 and some new extensions for $5.99.
DealTaker.com lists lots of Go Daddy and other coupons. Reg-Fee.com can be useful too. I have direct links to them on my 'Links' page at 'Names2Buy.com'. I don't make money on those links, to be clear!