Hello - I am an Au Pair :)
I look after 3 wonderful little girls.
I got my position through www.aupair-world.co.uk/
I set up a profile and had responses within the 1st hour that I had created it.
My advice being the one who was on the other end being hired is that we get lots of offers at the same time and chose our host family too, it's not all about family choosing an Au Pair. I had 3 families offer me a job on the same day - I Skyped them all lots but at the end of it, I had to choose the one who I thought were genuinely nice people and where honest, no hidden secrets and were themselves. Make sure they have a valid CRB check too! I can't count how many dodgy people could be on there. Make sure they have Skype so you can meet that way, or if they are close then get them to meet you for a coffee or something without kids so you can have a chat that way... makes it more personal.
Make sure you know all bout them and have read the profile thoroughly, I was let down 7 times because people did not read my profile correctly and expected me to be able to drive when I can't. The family I am with now have had several Au Pairs from abroad who don't speak English well, so I would make sure they do so that you can all communicate effectively. I am their first English Au Pair and they said it's a completely different and stress free situation - the kids aren't shouting to the Au Pair to get them to understand them and vice versa... the girls got so so stressed!
^ Someone above said speak to Parents - That's a good idea, unless they are old enough (One girl the family I'm with hired last year was 14 and running away from her parents claiming to be 18) So I would always be cautious and get them to scan in passport or birth certificate for evidence, as well as the CRB.
Make sure the Au Pair knows ALL the rules, don't leave things out. Get the kids to think of appropriate questions and get them to ask over Skype. As for contracts, if you search on google there are contract templates you can use/ modify to your needs.
I hope I helped :D
Good Luck xx