hideehi, coming from an experienced nanny, i've made a list of things to make sure she stays put!
If she's live-in, treat her as one of the family. Allow her to use the home as her own so she feels completely comfortable and occassionally ask her to join you in the evening when the children are in bed to watch tv, listen to music, whatever!
Don't expect too much off her with the housekeeping side, just childrens laundry/meals/cleaning rooms and playrooms in normal.
If you take the children out at weekends, ask her along sometimes so she feels involved.
Always try to get in on time so she can do as she pleased from the time she is supposed to finish
Hold regular " how do you think it's going?" meetings with her...just really informal sitdowns and smile alot and praise her so if she has a problem she can feel completely comfortable to let it all out rather than leave the job because you are unapproachable (which has ahppened before with me!)
Take her round the local area if shes new to it so she doesn't feel boxed in all the time and she can treat the area as home.
Thank her everynight before she clocks off and tell her how much the children love her and what a great help she is.
Give her a weekly allowance perhaps £15 or however much you can afford so she can take the children out to places or buy new things, such as ingredients for a specific dinner she wants to cook.