I do the same thing as purplefiend. Using my walking foot and a slightly bigger stitch, I quilt one line down the middle lengthwise and then widthwise. Before quilting these lines, I also use straight pins 3 or 4 inches apart all the way down the seamline to keep it taut and pucker free. I remove these pins as I stitch. This stabalizes the quilt and I can then go ahead and quilt each section. I also find that the batting makes a big difference. Cotton batting makes a smoother, flatter product with a smaller chance of puckering. I find polyester batts much more likely to pucker on the back, even if the quilt is pinned really well. If your backing is polyester/cotton it is also more likely to pucker or shift.