How to deal with mother in law?

I get along pretty well with my mother in law but she is so overbearing and stubborn. I have a 9 month old baby and she has an opinion about EVERYTHING. Its my turn to raise him. I dont mind her advice and help but there are things I want to do my way. The problem is that she wont back down. For example, she told me I shouldnt give my baby fluoride. My baby's dr said its important to give fluoride water but she says the dr's "dont know about health." I told her I trust my dr and would like to follow my dr's orders to give fluoride water and she argued with me about this. I said ok ok because I dont want to argue with her. Then she got mad at me and said I shouldnt say ok just to please her. SO WHAT DO I DO???? i cant win with this lady! Its the same thing with a playpen. she argued with me that i need a playpen even though i really dont NEED one. if i argue with her she wont back down and if i say ok ok she gets mad. How do I handle this???? She really drives me crazy.