A good thing to do in this situation is to put the fare on hold. If you want to buy a few days out you could go in each day and rehold (without AAdvantage number), then cancel the original hold that expires that day and then add your AAdvantage number to the new reservation.

Now, the important part is to look at the date that the new reservation is held until. If it is until 11:59pm on the following day, keep going for as many iterations until you want to purchase (or the fare increases). However, if the newly held reservation expires on 11:59pm on the same day, make your purchase that day. If you wait until the following day, this fare will not be available.

Be careful to check the "held until" date. I have got burned by holding a flight and presuming it is held until the following day, and then going back the next day to find my hold expired and the fare doubled.