Lots of good suggestions given already. Here are a few other things:
If your backing is pieced, make sure the selvage is removed and the seam is pressed (check if she prefers open or to one side). Also have the backing square - we can't load it straight on the frame if it's not.
Communicate your likes and dislikes to the LAer - we're not mind readers. Some people like dense quilting, others prefer looser, more open patterns.....some people love feathers, others hate them.....you get the idea. This will help you not be disappointed in the finished product.
Measure your quilt along the edges and also through the center - the measurements should be fairly close (no more than an inch or so different.) If your borders have more fabric than the center, we may not be able to quilt the top without pleats and puckers. If you need to know how to add borders so they are the right size, you can see my instructions here:
http://www.andicraftsquilting.com/learn.htm
Thanks for asking!