Make sure you find a girl who will like being in your house. ie- if you are a musical family or like football or needlework or go yomping every weekend, it will be a lot easier if you find a girl who also is, or at least appreciates these things or wants to do them.
Always remember that she is here on an exchange- she is not a domestic slave, but is here primarily to learn and improve her English. Helping you is her pay of paying her way. The pocket money is to make her experience of being abroad more enjoyable and enable her to feel independent. It is not really pay. The "pay" is the experience of living in a foreign family for a year or so and learning a new language.
Find out in advance from her what she likes doing in her free time, and what classes there are around that she may be interested in.
Locate your nearest and most appropriate ESOL classes and insist that she goes. I pay for my au pairs' English classes as I very strongly feel that ensuring that they learn English is very much part of my half of the exchange. You will probably, depending on her level of English and boldness, need to do all the initial step of registering her. Show her how to get there and take her there at least the first time and possibly a few more times.
Set aside at least a week or ten days to settle her in and mentor her when she arrives, and make sure she knows how you like things done. If you don't actively explain things, it will take a lot longer for her to learn them. Don't get cross with her if she does something that annoys you that you haven't explained to her. Make sure she fully understands what you want her to do!
Don't make unreasonable demands on her. She is not a domestic slave, but an enthusiastic and helpful visitor. Always remember that you are as much an ambassador for your country to her as she is for hers to you.
Make sure she is warm and comfortable especially at the beginning when she is possibly very homesick. I bought my latest au pair a warm dressing gown and slippers (just from Primarni, not expensive) and she wears them a lot and was very grateful for them as they don't really have them in Spain.
I have only ever had lovely au pairs, but I must say that the first couple of weeks are always trying. Even if you speak a little of the au pair's language, she will feel disoriented, possibly homesick and will need a lot of kindness at first and will probably be a lot more subdued than her true personality. Once she starts to get her bearings and understand more of what is going on, her true personality will start to appear. Good luck in your search!