I press my seams open, then pin both sides of the seam, very very close to the seam and right at the edge of the fabric, making sure the corner matches and the pin catches the seam allowances from both pieces. When I'm sewing, I slow right down, one stitch at a time when I get up to the seam. I make sure my needle has caught and is well into the pinned seam allowance, sometimes I will manually sew with the knob thing, to go over a pin. I don't pull my pins out until half of the seam is sewn. My corners are usually pretty good. Not always spot on, but very often!