We have been using au pair world for the last few years and have found it great.
One downside to au pair world is when you get too many applications - while there is a standard letter to send to any unsuitable au pair, I found it quite time consuming to e-mail all the possibles.
I find it much easier now that we only consider male au pairs (fewer applications);
you can also restrict the number of countries where you want your au pair to be from - we never include anywhere outside of Europe. Legally it's just too complicated, and now we just get au pairs who are German speaking (as my son does German at school).
I generally never contact an au pair first, I wait for au pairs to contact us - after all, a good au pair is one who shows initiative.
Once they have contacted me, and they seem good, I send a very detailed letter, outlining all the tasks, what is important to us, what is unacceptable to us (smoking, having people come to stay, being disrespectful etc) - I generally put down really strict rules: we don't usually stick to all of them (for instance most of our au pairs have had friends/family come to visit and staying with us), but it is much easier to relax rules, than to make them stricter.
Some au pairs do not reply to this first letter (presumably having been scared off...). The ones that do, will get another letter asking for references - again, it's a good test on how organised they are, and once the references have been checked by us, we start skyping with the possible au pairs - at this point, it's usually no more than 3-4 possible au pairs.
While we have had some problems with au pairs (some are better, some are worse), most are absolutely brilliant - they are young people who quite bravely decide to go to a foreign country, which I think is very admirable. I think looking at them as an older son/daughter, or nephew/niece, is a good way to approach it.
Good luck.