Q. "I want to fire my babysitter, but I'm chicken. She's sweet, but she doesn't do things the way I want them done. How do I do it?"
A. Have you ever told her that she doesn't do things the way you want them done? If not, that should be your first course of action. It's only fair to let this sweet person have the opportunity to make things right. If, on the other hand, you truly believe that your sitter is not a good fit with your family, then you need to find another person. Either way, you've got to pluck up your courage, Chicken Little, and make your feelings known.
I realize that this is as appealing as an enema, and about as comfortable, but there's no other way.
I say this from experience. When I had a full-time sitter, I always felt like apologizing when I gave any direction. Even little things, like asking her to remember to use diaper cream or to be sure to remove the rubber valve from the sippy cup when rinsing it out, were said with hesitation. So while I can't advise you how to be more assertive, I can tell you that feeling sheepish, yet talking anyway, always yielded a better result than keeping mum and feeling like a fool.