My 5 year old also has high anxiety. He is much calmer when kept busy because it gives his mind less time to think. His preschool teacher has been wonderful. She will take me aside when he has shown signs of stress at school and if he has talked about anything going on in his little head. We have a lot of toys at home to act things out, to pretend with, and if we let him play and play with him some he will start to talk about things in his head. When he does this, he relaxes more, sleeps better and is overall happier. Other suggestions which we are trying this fall is recreational sports. Playing on a team, soccor, t-ball, basketball or running will greatly help. It gives them something to stay busy with, and keep their minds from wondering. We are also working on the fact that it's ok to have angery or mad thoughts, but you have to talk about them, or let the other person know they have hurt you.
My son is not as anxious as your daughter, but these things have helped us a lot!