When I used to put both the top and the bottom of the binding on at once, my husband was the master of mitrering corners for me- I always made him fold them.
Now that I sew the back on first, when I get almost to the end, I sew a diagonal line to the corner, the break the thread, fold the binding back, and the over to the next side, and start sewing again. They always look nice when I pop them open.
On the front- it's all about pulling everything into place. I'd say on average I get 2 out of 4 corners truly mitred, and the others a bit wonky. It's a good day when I can get three of them to be truly mitred.