One of our guest speakers at a guild meeting said she presses her seams open instead of to one side and she lines up the seams and pins just a couple threads past the seam. She goes real slow as she approaches the seam and once she sews across it, she pulls out the pin. At that point the seam is locked in place.
She passed around some of her quilts and her corners were perfect. Not even a thread off. So I tried her technique and I am getting closer. Some of my seams are spot on and some are pretty close. That's good enough for me!