If you have a particular resort in mind, you need to watch and be prepared to pounce as soon as the AP discounts are announced. The cheapest rooms will go very quickly, often the first day, although there is often some availability in the more expensive categories for several days. For example, when the AP discounts for December came out last year, many people wanted Wilderness Lodge, and there was no availability by the middle of the first day. Even people who had reserved a room ahead of time, hoping to convert it to the discounted rate, were disappointed. I read thread after thread of frustrated people who couldn't understand what happened. So be prepared and be persistent.
If you are flexible, it's much easier. If you can take any Epcot resort, instead of just one specific one, or if you can take any monorail resort, instead of a specific one, you stand a much better chance of getting a discounted room. I do a lot of last minute, long weekend, AP discounts, and I dont' have trouble getting a room. It may be my second or third choice of resort, but really, any Disney resort is going to be great.
You don't have to have the AP to make the reservation. You just need to have one at check in time. So I would wait and see if you can get the room before investing in the pass, if that's your main reason for getting one. After all, there are no guarantees that there will be any AP discounts at all.
