When we last went to WDW (from Amarillo, TX) in August of last year, I booked with Delta 7 months out, paying $652.80 for the two of us. I went ahead and booked amidst all this terrible news that airlines were raising rates and fuel costs were escalating and general woe to the airlines stuff. Then, about two months before the trip, we wondered if it would be possible to change our itinerary to arrive a little earlier. We had upgraded to the gold plan and knew time was money. Anyways, I was able to change our flights for a fare that was then $100 cheaper per passenger. After the $100 per passenger change fee, it was a wash. We did arrive earlier, though (in plenty of time for Hurricane Francis).
If I had waited until two months before, I could have gotten us both down there for around $450. I don't know that this sort of price decrease will happen again, but I'll be waiting to book until a little closer to time.