With big asses, you have to spend appropriate time down there - licking it, sucking on it, rubbing your face in it, opening it up and observing its photographic beauty. You have to tap your penis on it, squish it together etc.
Make sure you have a plan of action if you do book a girl with a big ass. At $4 - $5 per minute, it isn't smart to waste time by not having a plan.