There is always every month a full page of quilting tips published in the Quiltmaker magazine. It is worth the price for this one page alone. Long ago I ran a copy and put it in my sewing room where I can quickly lay my hands on it. The page shows you very clearly how to get perfect corners. The best part is the trick for joining the 2 ends with a bias seam, I have to look at it every time, but I love it. One thing you must do is press your binding first and be very careful to maintain a consistent seam, especially at the corners. You might also try reducing the downward pressure on the presser foot, and if you are not using a walking foot, you should.
HTH
Pat