AAA typically costs around $50 for a year long membership.
So whether or not it would be worth the discount depends on which resort you want to stay at and how many nights you are staying.
Occasionally, you may find a good deal through a particular
travel agency, but it's very rare that these will be better than what you can get directly from Disney.
Here are the most common discounts that you can find on Disney resort rooms:
AAA - The discount amount varies depending on which season it is and which kind of resort you want (value, moderate, deluxe). It ranges from 10%-20% off and it is almost always available.
Annual passholder - To get these, you have to have an Annual Pass, but sometimes they are good enough discounts that it's worthwhile for one person in your party to buy an AP just for the discount. Last spring, I bought an AP for this reason. It cost me about $230 more than a 7 day ticket, but it saved us about $500 off our rooms for our 2 week trip.
Florida Resident - Usually similar to the AP rates, but of course, you have to be a FL resident.
General Public - Disney runs special promotions and puts out discount codes from time to time.
Typically, you can get the AAA rate far in advance. The other discounts are usually available a few months in advance. For instance, right now, there are AP rates, FL rates, and general public room and package discount codes available through December.
If you know when you want to go and what you can afford, your best bet is to go ahead and make a reservation now, then keep you eye out on these boards and on Mousesavers for any news of a discount for your time period. You'll see that most upcoming months have a code watch thread here. Those are full of all the people who have trips for that month and are waiting to see if there will be any discounts.