With the head shaking thing I would assume that she's bonked her head really badly before. Firstly I would make sure that she has one of those head bumper things for her halter during trailer training and leg wraps. I wouldn't suggest shipping boots because they are a little looser and could get tangled.
Anyway...what I would do is...if you can get access to the trailer for like a month... Feed her a little grain and grass hay on the edge of the trailer. Then slowly move the hay farther and farther in. (MAKE SURE THE TRAILER IS HOOKED TO A TRUCK!) I did this with a 2 year old who had never been in a trailer before. It got to the point where she loved the trailer so much that she was annoying about it. The owners had the trailer parked in a paddock area...and when I would be cleaning it out she would jump in there with me and start searching for food.