Definitely don't press open seams, have the top seam going toward the machine and the bottom seam going toward you so the machine pulls/pushes them together tighter. But, I think pins factor in also. If you are using the ones with the yellow ball on the end, they are too thick. Several companies, I know Dritz is one, make glass head pins. They take less "bite" out of the fabric and the pin itself doesn't cause shifting.