Go with washed hair.  It's easier for the child to be able to just hop up in the chair and get going.

Let your child know that the hair will grow back and that cutting hair doesn't hurt.

Make appointments when your child is most likely to be fed, awake and happy. A tired, hungry child is less likely to sit still.

Encourage your child to watch someone else getting their haircut first. Go a few minutes early to the appointment so they can look around a get a little more comfortable with the surroundings.

Make it a part of a fun day. Go out for lunch, a haircut and then to a movie or the park.

Positive reinforcement. This is my favorite one. "What a big girl!" and "You are such a good boy!" will work wonders. It makes them feel grown up and they tend to concentrate on how well things are going.

Do bring a spare set of clothes just in case your child finds it uncomfortable with the hair trimmings.