First of all be clear about what you expect. If you are woolly, they will not have a clear idea either and will start doing things you don't like. I didn't make things clear with the first AuPair and came home to find a boyfriend ironing for her. He also eat food from the fridge. Oh no! So here is a list. No smoking by them or their guests in the house or grounds. Give VERY clear and explicit instructions about their duties and when they can take time off. Your children do not enter their room without knocking and visa versa. No boyfriends/ girlfriends staying the night.
I would write instructions for them, as on Monday this happens., Tuesday this happens etc. Best of luck as I didn't have AuPairs until my children could speak. They are going to surprise you!
We are however still in touch with some of ours and they were on the whole a lovely bunch. All Germans apart from one Italien. I made sure that they had excellent Abaturs and had siblings. I found the more intelligent they were the easier they found it to adapt. Also they had to be between school and University for me. I never had a bloke.