Personally, I find that spending at least 1 day with my new new AP guiding her through a typical day is very useful. This helps her become aquainted with our routines, and is a good indicator of what she is expected to do during a typical 'working' day.
Don't take any knowledge for granted - so show her how to work your household appliances, write down portion sizes for your children, your house rules, emergency numbers, where food is kept, the water stopcock etc. Remind her of the kind of activites your dc like doing.
Also it's nice to take time to show her around your local area. Show her the nearest shops, gym, parks, bus stops, train station, doctors, dentist etc. I would also show her how to buy a train ticket too if your station has an electronic ticket machine.
In our welcome pack I include a doctors and dentist registration form, train timetables and a local map and also a London A-Z together with our contract. I also include a daily checklist now so that it is easy for the AP to know what she has to do. It can be quite overwhelming for the AP at first to remember everything.
Most of all make time to talk to her regularly, and find out how she is doing. Address any problems immediately.
Having said all of this, I still think that having APs are very much a lottery. You will pretty much know whether you have a good AP within the first week.