I believe there are at least 3 things to watch for nice mitered corners.
First, the quilt must be properly trimmed.
Second, avoid sewing into the 1/4 inch corner (if sewing 1/4 inch binding)
3/8 inch (if sewing 3/8 inch binding). Slow down and stop before that
mark.
Third, when folding the binding, make sure it's flush with the binding.
I know we all have different ways of sewing binding. For me sewing
to the corner never worked. There must be a secret to that that I
haven't found yet. So I prefer to stop before that corner. And when
I resume sewing the binding, I put my needle at the 1/4 or 3/8 inch
mark then backstitch but not all the way to the fold. That's the key.
You just have to pay attention to how you are sewing and note what
works for you and what doesn't. That's how you can improve.
I use Patrick Loose's tip and trim the binding in the corners.
This helps with the bulk.