Why do you have to wash her hair? I think that you're in a power struggle. Washing hair has become a big issue, apparently for both of you.
I recomend letting go of having to wash her hair. Don't mention it at all. Wpn't she be surprised. It may just cause her to go back to washing hair. Another technique you could use is to tell her she doesn't have to wash her hair. Be cheerful like it's no big deal.
I've learned lots about discipline from a book named Love and Logic by Foster Cline. One of the ways to avoid a power struggle is to stop yourself from fighting. And try a different approach.
My granddaughter has thick curly hair. She started crying when we had to brush her hair. It truly does hurt. She's 8 now and can brush her own hair. somehow it doesn't hurt when she brushes. She let her mother brush her hair after she found a way of brushing that is more gentle. One thing that she did was to spray her hair so that it was halfway wet. My daughter takes one strand of hair, about 1-2 inches and holds it close to the scalp while she bruses down. She does this around the head. Then she brushes the top so that all the hair blend in.
I use a wide tooth comb which also has large teeth for my hair when it's wet. The comb goes thru more easily.
My daughter also let her watch TV while she brushes her daughter's hair. This distracts her so that she's not so aware of what is going on.
Neither my daughter or I go for prefection; not even close. Brush it quickly. THen let her decide how she would like to wear her hair today. She will be more cooperative when you're doing the pony tail she asked for. Possibilities are braids, either one or two, You can put her hair in two pony tails on the sides. How about a palm tree in which the hair is put in a pony tail on top of the head.
Perhaps she would be more interested if she could pick out the barretts, ribbon or headband. make the choice simple. My granddaughter's hair accessories are in a plastic bin. She then chooses from the bin. If you want more control you could have 2 or 3 out and let her choose from those.
A reason that she might be rebelling about the hair is that she doesn't like her hairstyle. Keeping her hair short does make taking care of it easier.
Involve her in making choices.
Another thought. Let her put the sampoo in her hair and squish it around. You could make it a game. She gets the shampoo started and you as the "hair inspector" will finish it.
I've been thinking that you're washing her hair while in the shower or tub. I think it's easier to do so. When my granddaughter was that young she showered with her mother. They sang and played around. They had tub crayons to draw on the surround.
Again you don't have to shower/bathe everyday, nor do you have to shampoo her hair. Take the pressure off your daughter. If she says she doesn't want to do it this day then say OK but we must have an agreement that you will tomorrow.
You could write the agreement down and post it in the bathroom. The idea is to give her as much power that she can safely handle. This is the age at which we parents begin the process of training our children to be independant.
She could brush her hair with you completing it. She could not have her hair brushed at all. She definitely doesn't need a neat hairdo.
Find ways to involve her and keep the tone light.
Some kids respond to singing. Sing the instructions. Now is the time to get in the shower, etc.
ADDED 8/21/08
I just read an article in the First magazine which is displayed at the grocery check out. The person in the article said that her beautician had recommended the following so that she could brush out her hair more easily.
The woman said she has long, thick curly hair. Her beautician suggested that while the hair is still wet use her hair dryer on cold setting and while blowing the air on the air to run her fingers thru it. She said she has no trouble with tangles now.