I agree with Lucy In the Sky...invest in better pins, those are too thick. You might try spray starch and pressing. It's almost as good as basting. Then with fine pins, you can poke right on the stitching at the top seam, 1/4 inch down from the top and through to 1/4 inch from the top on the back and make a small stitch with the pin. Your seams should be pressed so that the nest against each other and if possible, sew in the direction to force the top seam against the bottom seam. I find this more accurate than pinning on each side of the seam.