Where abouts are you? Obviously training is paramount but as you've booked the work in, I'm guessing you're just going to crack on anyway. I would say the biggest thing is to avoid over wetting, running the colours, ripping the material etc etc
You really should be utilising at least one air mover and keep the psi down depending of course on which upholstery tool you are using! Take your time, if you've done a 3 piece suite in under 3 hours, you're doing something wrong. I take my time on sofa's as it's very easy to miss something if you're rushing! + I actually enjoy cleaning upholstery, I find it theraputic!!!!
The reason I ask where you are is because maybe someone would show you exactly how it's done and avoid you having to spend a fortune on a new sofa if all goes tits up!
I wouldn't want an untrained C/C cleaning our sofa.... It cost us the best part of £3k!