Try glue basting at the ironing board before you take your pieces to the sewing machine. First step is to push a straight pin down into the seams that need to match, right at the seam allowance. Once you are sure the pin is positioned so that the seams match, lift up the seam allowance on the top one and insert a tiny dot of Elmer's washable school glue (white). Close the seam and press for a few seconds with a hot iron. Remove the pin.
I find glue basting more accurate than pinning.