You have to decide what kind of au pair you would like. Younger (ie 18-20) vs older (over 25), from Eastern Europe, including Accession States vs Western Europe, and a career minded person vs someone who is drifting through life. Each choice is very personal and has its benenfits and drawbacks.
You also have to decide what level of English you can put up with, whether you need the au pair to be able to drive, and how long you want them to stay with you.
My own point of view, based on experience of 3 au pairs, is:
- to get someone younger since they are more likely to be energetic and willing to muck in;
2)a western girl, because you shouldn't have to explain as much
3)someone who is about to go to university so that you can maximise your chances of getting someone who is bright and has initiative (the downside of this is that they are likely to get bored and move on after a few months)
4)to get someone with a moderate level of English - bad enough that they qualify for free classes, and good enough that it is not too stressful to communicate. They need something to do during their free time.
- to have the au pair stay for 3 months (school terms, basically) and that way they stay keen and interested.
An agency costs about £300 to place an au pair and it's not really clear what they do for the money. You can DIY by using Great Au Pair or Aupairworld.net. I've used both and prefer Aupairworld, because their matches are more suited to my needs, and it's cheaper. GAP is a great site though, and it's well worth registering with each - you only pay the fee when you want to contact a prospective au pair.