My son used to be this way with the dentist, and with brushing his teeth everyday. It was such a terrible time of the day, and he threw up every time. Anyway, then I took him to a pediatric dentist and he did great. They told me to stay in the waiting room, which I was very skeptical about, but I did. To my suprise, he was perfect and didn't cry except when they were taking him back. My heart sank, but when I found out it worked well, I was thrilled. So, try taking him to a pediatric dentist. If that doesn't work, try another regular dentist. It may just be she doesn't like that one you go to.
On a side note, my son hated getting his teeth brushed. After we went to the pediatric dentist, I hunted on line for a new toothpaste to try to make our experience better. We had tried every toothpaste at all the stores, and he hated it. Well, I researched candy toothpaste and found Tanner's Tasty Paste which is toothpaste that tastes like ice cream. I bought some, and he loves it. He no longer cries or vomits when I brush his teeth. That may have helped with him liking the dentist too, since our home experience is much nicer now.
To help her prepare, just tell her the dentist will be tickling her teeth.