I don't bother checking references - they are no guarantee of anything.
I get my au pairs from aupairworld. I've had a lot of success from there and it is cheap and easy to find an aupair.
I wait for aupairs to respond to my profile and then I will give them a yes or a no. I then wait for them to contact me. Usually, they send a chatty email, telling me about themselves and asking about our family and the job. I have a little webpage with more details, including local links, that I send them, and will add to it if I am asked a question I haven't thought of before.
The key thing, IMO, to decide up front is the age of the girl (eg at the younger end or at the older end of the au pair spectrum), the level of English that you require, specific skills like being able to drive, food issues, type of family background (ie number of siblings). Then you can think about things like hobbies and ambitions - it is good for them to have something to occupy themselves with as being an au pair can be very lonely at times, especially if she is only working 25 hours a week. One of my best au pairs was a girl who loved running and practicing her violin - she never got bored.
YOu also need to come up with a list of hours and duties, so that there are no misunderstandings later. Money too. You should also decide how long you want her to stay - you get a lot more mileage out of an au pair when they are new, so think about getting a new one every school term rather than going for an indefinite one.