Make sure your back is bigger than the top, usually 3-4 inches all the way around or 6-8" longer and wider. We use this area to attach the back to the leaders on the poles. We also use the side areas to test stitching, making sure the tension is good. And even though I don't ask for them, I really like it when scraps of the actual fabric are sent too. Sometimes there are fabrics that don't play well, and it helps to narrow down problems if they occur.
Before you go into what you want done, let her know the budget. Measure your quilt, then multiply the length by the width to get the square inches. Then take some rates, .015 (which around here is an allover panto price), .02, .03, up to that point that you just can't go beyond. That way you won't fall over when she gives you an estimate, you'll be somewhat prepared for the range of possibilities.
After you've had a few quilts done, or if you've seen lots of a quilter's work and are comfortable, it can be fun to just turn her loose and see what magic she does.