Not sure if this has been discussed befor but when we booked our AA tickets back in May for the flight to SXM July 31, we got good seats on all flights except PHL to SXM. We had to take center seats or pay $50 to $100 for the "Better Seats".

I started googling and found that AA saves a lot of seats to the last minute for their frequency fliers. It turns out that they release all the seats 24 hours before boarding time.

Sure enough, checked back 24 hours ahead and there were all sorts of seats. Most passengers had an empty center seat between them when we boarded.