We went through this and still do with my now 7 year old. I talked with my doctor, a very laid back easy going family doctor who we all go to and love very much. He, a father of four, always steps back and looks at the big picture. His first question was who in your family acts this way. Must have their way or throws a fit. For us that was pretty obvious, my mother, sister and even myself. We like to be in control. The second thing he asked was how disruptive is it to our family to do as she asks? Is it worth the battle? The third thing he said was to make sure and include her in the conversation and let her have the choice. When you know you are about to do something that normally sets her off, tell her, give her the choice to come and help.
For us, the big issues were in the bathroom, the van and the TV. The bathroom had to be cleaned her way. Ok clean it, flylady says even cleaning done wrong is a blessing to your family. We gave her responsiblity of cleaning her bathroom at about 2 1/2. I go behind her when she is distracted and really clean, but she thinks it is still done her way.
In the van, the tv screen had to be up before she could get out of the car. Again we gave her control of turning it off and pushing it up. The extra few seconds were not that big of a deal.
The TV at home was our biggest issue. She could not turn the tv off in the middle of a show. wasnt happening, dont ask, melt down was coming. After fighting with her for what seemed like weeks, a light bulb went off and we started telling her when we were leaving, and that at the end of XYZ the tv was going off so we could get ready to go. If we didnt start the new show, we never missed what was going on. Presto, no more fits leaving the house. Sounds simple, but it took a little planning and many fits before I figured that one out :)
She still has her issues, but the fits are coming fewer and fewer. For us the love and logic system worked wonders, check it out and see if it helps.
Good luck!