I also stick a pin directly in the seam of the top piece, then make sure that pin is straight down into the seam of the under piece. Make sure that pin is as perfectly perpendicular as you can get it. Holding this steady, then place pins on either side of that seam as close to the seam as possible and remove the perpendicular pin. I find this helps greatly in getting my seams/points to match up and stay together during the sewing process.