I don't think there's a AAA discount for United. You might be successful in booking some kind of vacation package cheaper, but not just flights. For cheap flights, all you need is time and FT
For everything else (hotels, cars...) I've always gone directly through the website of whoever I was booking something from. I've gotten excellent deals from Starwood for example: 10% off their best advertised deal, without knowing what the current promo is ^