One of the "rules" for corners is that you *must* stop the exact seam allowance that you are using. For example, if you are sewing your binding on with 1/2" seam allowance, you must stop *exactly* 1/2" from the edge (and backstitch to secure). If you sew binding on with 1/4" seam allowance, you must stop exactly 1/4" from the edge.
Until I realized this really is a rule, my mitered corners were terrible. Nowadays I mark the stopping point and then make sure my needle enters that stopping point and is not one stitch over or under. Really makes a difference!