There's an article by Kim Brunner in the latest issue of Machine Quilting Unlimited called Taming an Unruly Quilt - Eliminating Fullness in Blocks and Borders. She talks about this problem and how to deal with it. I just scanned through the article (but will read it in more detail later). She didn't use extra batting, but she did anchor with pins, smooth the top out carefully, take care in loading, emphasized keeping the lines of the quilt straight. I learn a lot from each issue of this magazine and highly recommend it for longarmers especially.