DOConsider carefully if your family can cope with a stranger living in the house all the time.
Go through an agency if you want the safeguard of better vetting and backup if something goes wrong.
Write down what tasks you expect him/her to do.
Establish the house rules, such as if s/he can have a friend around.
Give him/her a weekly schedule so s/he can plan her own spare time.
Remember the first few weeks can be difficult not only for your au pair but also for your children, so be patient and talk through any teething problems.
DON'T
Ask an untrained au pair to care for a baby alone.
Treat him/her as hired help - instead treat him/her as a member of the family.
Burden him/her with hours of housework.
Expect your au pair to stay in her room if s/he is off and at home.
Leave him/her alone with children overnight or for a weekend.
Put up with him/her if you cannot stand her, a clash of personalities is not good for either side.