You have to visit mousesavers.com! I saved $700 on our reservation AFTER we had already booked! I called WDW reservations and modified my existing reservation using the appropriate code, and the CM quoted me the new rate - Wow! My husband thinks I am the greatest
There are also tons of other ways to get discounts in the parks, airport transfers, etc. There is just a lot of good information. I will advise all my Disney-bound friends to go to mousesavers first thing!
P.S. I had planned on getting an AP to get the best deal, but because of the code, I didn't have to. Best advice is to go ahead and book the room/cabin you want so you'll be sure to get a reservation, and check mousesavers often to see if any code you can use comes up. Then you can call back and "modify" your reservation. As a last resort, you can get an AP if nothing better comes along. Good Luck!