Most nannies I know look on nannyjob.co.uk, even if they aren't actually looking for a new position!
I think it costs about £20 to advertise.
Gumtree is the next site I go to, to look. I don't know how much they charge to advertise.
School/nursery/community Center notice boards. Local newsagent window.
Try to add as much information as you can. Make sure all your information is correct before posting, I've replied to adverts and find out the key things I am looking for aren't correct!!
Location is important, don't say the county, especially if you are looking for live out. Say x miles south of main town rather than near main town, it can be 20-30 miles from one side of the town to the other adding an hour each way to the day.
Ages and sex of children.
Duties.
Salary range. I don't answer adverts if there is no salary range, I am really, really bad at talking about money, face to face or over the phone. So are a lot of nannies I know. There is no point going for an interview if you can't afford to work for what the family are offering.
(Before deciding on what pay you can afford, don't forget to factor in activities/outings for the children, food for nanny (just basics, whatever the child/ren are eating during the day, if your nanny starts at 7am and has a 30 minute commute, she'll need breakfast and lunch, if she works until 7pm she may want tea too! I know that eating at 8:30/9pm is too late for me when I have to be up at 5:30 to get ready for work) milk for her tea/coffee and biscuits! I get through a lot of fruit! I do provide some of my own fruit as I eat loads. I also buy anything out of the ordinary for me to eat. I like salmon, my bosses like the children to have organic meat/fish (I do too!) so when I do the children's food shop, I pay for mine. But I will use their veg, carbs and store cupboard ingredients.
Also, fuel/transport costs for activities/outings/school. 45p per mile is the current rate for a nanny using her own car. The odd tea/coffee/cake/ice cream at soft play/park/town when they pop in to buy new shoes/clothes/etc for the children. My ex-charge had two classes that my boss booked in town on a Tuesday, we didn't have time to go home for lunch in between classes so we would have lunch in town and my boss would pay as it was her choice, if I choose to have lunch in town then I would pay for myself, my charge was 6 months so I took food for her. It all adds up unfortunately! Sorry, slightly off topic!)
Good luck!!