Check your library or quilt guild to see if you can hunt up a copy of Linda Taylor's "Ultimate Guide to Longarm Machine Quilting: How to Use Any Longarm Machine Techniques, Patterns & Pantographs Starting a Business Hiring a Longarm Machine".
Lots of good info, but it looks like it's out of print.
A form should include: today's date, both names & phone numbers, quilt size in inches, description of quilting or name of panto, price per sq inch, thread to be used (color & brand), who supplies the backing and batting and prices if you are, fees for additional services (seaming backing, pressing, etc.), approximate date of completion. You should also have an estimated total cost (I'm amazed at the number of folks who don't realize how quickly $0.02/square inch can add up). Both signatures.
I would also suggest that you spend 15 minutes or so with the client when they drop the quilt off. Open it up & look at it. If you have room, lay it out on the floor. Look at the backing. I worked with a long arm quilter who insisted on doing this. A few minutes up front can save a lot of headaches, although she had customers who just wanted to hand her the top, pick out a panto and leave.