I'd go along with what everyone has said so far.
Don't be late, unless unavoidable. Text or phone to say you'll be late and apologise when you get in. Offer to pay overtime for the extra time worked. If you're treating your nanny well he/she'll say 'no, it's ok'.
Always say 'thank you', it goes a really long way.
Be as generous as you can with wages/holidays.
Remember that the end time on the nanny's contract is the time she leaves, not the time you get in. If you want time to chat at the end of the day (a good idea), get in 10-15 mins before she is due to leave. Same goes for the morning.
Don't micro-manage, but at the same time make it clear at the outset what you require - she can't do her job properly if she doesn't know what you want.
If she asks for stuff for the children, or to take them somewhere, or have people over for a playdate agree wherever reasonable (ime being laid back about this sort of stuff earns big brownie points).
If she is off sick try and be concerned and say 'hope you feel better soon'. not 'well what am i supposed to do? I've got to go to work!' (a friend of mine had this a couple of times with her boss!)
Keep the lines of communication open so if she does have a concern/problem then she can tell you straight away and you can sort it out together. An annual review is a good idea.
I think having a really good contract in place at the start is essential, so everyone knows exactly what they need to be doing to help the other person. Also if you're very keen on them staying ask at the interview somthing along the lines of 'what are your plans for the future/next 5 years etc?'. Don't want to take someone on only to find they're planning to go travelling in 18 months time!
I've been in my job for 2 years and have every intention of staying until the youngest is at full-time school (another 2 years at least).
I'm in London, but I'd say the normal/good wage here is 10pph net.