First, I normally press my seams open. It makes things lay flatter. Then when matching seams. I put my pin in the center of the top seam, perpendicular to the fabric and pin straight down. Then look at the bottom seam, put the pin straight through that seam - again perpendicular to the fabric. Leave that pin sticking straight up and keep as straight as possible. Then put pins in normal fashion on either side of the perpendicular pin and then remove that pin. I also sew over my pins. Doesn't always work but does more often than not.