As promised, a list of my complete cock-ups! And here's what I'll do next time (if i have an au pair again)
Duties: I'll ask for more than I think I'll need. I didn't want housework done as part of the 'job' initially, but now find the kids' schedules mean there are 90 mins where she effectively does bugger all because they're asleep. Backtracking on 'no housework needed' is difficult.
Respect for 'my' time: I have been really careful not to intrude on her personal time. e.g. I don't ask for her to be up at the crack of dawn to assist with school run if she is working in the afternoon. I think, in her mind, this translated into her time being more important than my time and so social engagements soon started to cut into her work hours.
Food: I will NOT be as accommodating. I assumed someone who wasn't fussy about food would be a dream. She loved vegetables, spices, stir fries.
Had not anticipated her adopting a no-carb diet and refusal to touch any pasta, rice, potatoes, root veg and dismissal of anything other than fish or lean (white) meat. (We normally eat pretty low carb, but food bills were astronomical initially).
We now cook what we would normally cook, it's all fresh, healthy and well-balanced, and she eats it or leaves it.
Oh, and accept you may be used as an eat-as-much-as-you-can buffet.
Kids activities: unless they've worked in a nursery environment they may be clueless about what activities to do with the kids if they are stuck at home. If you don't want the TV on, say so and draw up a list of suggested activities.
Recycling / energy conservation, etc: ours wouldn't use the washing line in the height of summer, tumble dryer was on constantly. 30 minute showers...we're on a water meter. A young girl has no idea about cost of living or environmental issues (unless they are north european) so you'll have to explain.
Pay - she'll talk to other nannies about pay, and may feel she's being ripped off/exploited. Be clear in your own mind what the difference between an au pair and a nanny is.
Babysitting - schedule your day(s) otherwise they may disappear when her social life hots up. It's a pain in the arse having to go cap in hand to your au pair and ask if you can go out tonight.