Well a lot depends on the actual arrangement (sole/shared charge, driver or no, part/full time etc.....) as a nanny I would say-
Do-
-Communicate! Tell nanny when things are wrong, or if you are impressed with anything. Be clear if nanny can have other nannies over, if they can take your DCs on playdates etc.
-Remember they are a person, and may not be perfect. A bit of understanding if things go a bit wrong is appreciated.
-Be very clear what you expect from them, and remember how you do something is not how everyone does it. Many times had incredulous parents who can't understand that I do something like folding clothes or cooking chicken different to how they do it.
-Keep up with kitty. Nothing worse than having to either cancel something or fund it yourself and hope for the money back.
-Be friendly. A simple 'how was your weekend?' or something is nothing really but I've had two jobs with no conversation beyond do this, do that and it gets lonely!
-Be clear and upfront if job has unusual aspects. One family had a second home and expected me to travel there on my day off each week- if I had known at interview, wouldn't have taken the job.
-Be understanding if they have to have a day off if they catch an illness from your own children!
-Offer perks! My favourite bosses are those who let me borrow a book or DVD, give me leftover lasagne
and my current family let me come over and watch their TV if they are away for the weekend (they know I don't have one). Little things add up!
Don't-
-Tell them they can help themselves to anything they want for lunch, or use anything from the cupboard/fridge for the children then get annoyed they use something you wanted. We can't read minds!
-BE LATE! Well, without telling them or texting/calling as soon as you know. If late, be appologetic. A sorry goes a long way! Don't think that if they don't say anything, or say 'oh its ok' if you get in late, that they are fine with it.
-Expect nanny to babysit with very little notice. If I have nothing else on I say yes but when I can't and my boss gets annoyed, even if they don't say it, I just feel guilty and I hate that!
-Assume nanny will be happy to just look after the children of friends etc without telling them. Extra children is extra work and means scrapping plans and having to change meals etc. A little notice is appreciated!
-Undermine nanny in front of DCs. Back them up, if you think how they are dealing with something isn't how you'd prefer then speak to them in private.
-Be dismissive of their job/career. Don't call them the babysitter!
I'm sure there are many more things though!