We're an experienced host family and have had a number of very successful au-pairs. We've also had a couple of disasters too. As OVienna correctly states - there is no magic formula at all. It is due to luck. Even with many interviews, references, emails etc; until they actually arrive, and spend a few days with your family, will you actually know if they are any good or not.
We always invite them over for a trial weekend (which we pay for), and if there are any doubts, we cut our losses. if a trial weekend is not possible, I make a decision within three weeks of their arrival whether to keep them. Unsuitable au-pairs rarely improve over the long term, and usual revert to their bad habits quickly.
I never use agencies any more. They are a waste of money, and are purely concerned in placing au-pairs anywhere as they will be paid both by the host family and the au-pair. I don't believe agencies are acting for the welfare of either the family or the au-pair. There are plenty of enthusiastic, experienced girls on Au-Pair world - just be sure to ask for copies of ID, police check, health certificate, job references.
My loose criteria I use when sifting through the many applicants on APW is:-
Experienced with children - ideally babysitting as the skills involved here are very similar to what I need
Actually enjoy being with children (only can be evidenced on the trial weekend or when they arrive)
Have patience with children (again only can be evidenced in person)
German/Austrian (Due to excellent standard of English)
Gap year students (Tend to be still in school mode of being quite conformist and actually more sensible than new grads).
Must have worked in some capacity (and therefore have a good work ethic)
Have a sunny personality (makes a real difference as they will be living with you)
Finally, i try to remember when recruiting and wading through the applicants, you have to kiss many frogs before you find the one!