Start calling about three months prior to the month of your arrival (as Disney Traveler said, October). Simply ask for Annual Pass discount rates for the dates in question. If they're available, these will be treated as a new reservation--they'll credit your current room deposit to your new reservation. Keep calling back if you don't get a discount--they often release new AP discounts into the system.
You'll be going during one of the better periods for AP discount availability, so you stand a reasonable chance of getting a discount of some sort. Be open to switching resorts. Annual Pass discounts are traditionally better (and more freqeuntly available) on the Deluxe resorts.
Also, sometimes a discount may not be available for the entire duration of your stay. If they don't have discounts for your intended dates of arrival to departure, try shaving a day off the front or back of your schedule, and see if any new availabilities pop up. You may have to pay regular rate for a day or so you can get a discount for the rest of your stay.
They say the vouchers arrive in something like 3-4 weeks, but you don't have to have a pass or voucher in hand to reserve an AP discount--you just have to be ready to show your pass (or voucher) when you check in to your resort.
You do need a bit of patience with AP discounts, but the work can pay off nicely.
Co-Author, PassPorter Walt Disney World 2000 http://www.passporter.com/wdw/