I dont wish to be controversial here - and god knows I'm all about the orgasms - but I dont think you should start trying to work on your ability to cum during bookings.
It's something that you need to explore with someone who is there for your mutual pleasure ie partner, friend, fuck buddy, whatever. That way you can really relax, and discover what it takes you to get you to where you want to be.
I'm not saying that you shouldn't enjoy your bookings, or that your Clients dont want or wont enjoy you orgasming, just that it will be far easier to explore if you're not mentally keeping an eye on the clock, that he has his condom in place, that he's not scoping your room to steal your money, that house keeping aren't about to knock on the door, that your next booking wont be early and all the other million and one things that occupy a working girls mind!
R xx