Here's something I've been in the habit of doing ever since my kids started school:
When you see excellence in a teacher ALWAYS write to the school board, principal and the teacher commending them. Too many times parents only get involved when they are complaining. Take the time to let your school system know when they have excellent teachers.
This is of course all filtered by your child's definition about what is excellent. So if your child likes a teacher then you write a letter about how great the teacher. If your child has a conflict with the teacher then they are bad and nasty.
I would be very careful here with under what basis is used for your praise. As I posted above a good Principal knows the good teachers but there are also teachers the kids love who do no teaching at all.
The popular teachers offer a lot of esteem building excercises but little learning.