I was the same with my first customer too mate, its gets easier. As much as you practice on your own windows, its also about getting use to all the different types of windows.
A few tips:-
wear shoe covers inside, especially on cream carpets
if the window turns in, put ur bucket under the window, or take a towel with you.
Internal ladders are needed some times, I had to use my external ladders inside a converted church.
I like to be very curtious inside people's homes and always ask to do things first like, go in a room with a closed door, I think it puts people at ease, especial old dears in on there own, so be a gentle man basically.
Common sense, but none the less hopefully useful tips.