I also use large office clamps when I lay out my backing. I clamp it to a table, then add the batting and clamp it. Then lay out my quilt top on it. I also use quilters safety pins and pin about every 4-5 inches. Since using the clamps I haven't had any puckers in the back of my quilts - and I have done small sizes and large sized quilts. I spray baste my small projects, usually not my large projects. Keep trying, you'll get it!!