First, I would definitely think about the areas where you are absolutely clear on your opinion and that are nonnegotiable to you. People differ on many things and you don't want a misunderstanding about expectations to disrupt an otherwise good childcare arrangement.
Be it which types of foods are off limits (choking hazards or allergies), what you deem unsafe (cooking on the front burners, kids not in car seats), dispensing of medications, over the counter or prescription (are any ok drugs provided allowed without her asking you first?), should she have child CPR and first aid certification (she may ask that you pay for the class), appropriate discipline (time outs, what else is ok).
Second, be very clear about work items like in any job. THe hours expected to work, pay per hour or week, how you'll deal with overtime (>40 hours per week), whether you are paying her taxes or not, what is the expectation regarding notification for taking time off (sick or vacation), what happens if she is running late, what happens when you are running late, is "babysitting" on the weekends possible if separately scheduled, when you take a day off do you pay her for not coming to your house (on this I say yes unless described as unpaid time when you first make your agreement), etc.
I had a nanny until my husband decided to stay home due to a company change. I loved her and we still see her regularly. However, it was tons easier to talk with her about things that we discussed up front than about issues that came up later than I sometimes felt "silly" talking about.
Good luck!
Ruth