It is important to have a consistent seam allowance and stop sewing at the corner the distance of your seam allowance. So many people say stop 1/4" from the corner. That only works if you are using 1/4" seam allowance to sew your binding on. I use 3/8" seam allowance because I use my walking foot and the width of my waking foot is 3/8" and so I use the edge of the foot as my seam guide. If I stopped 1/4" from the corner my miters wouldn't work either.
Secondly, I don't press my bindings before I put them on. I feel that they just handle more easily if they are not pressed. I also hand stitch my bindings on the back so like the looser fold to hide my stitches in.
Thirdly, when I am mitering the back I make sure to lay the binding flat all the way up as far as I can. It needs to be flat at the edge of the quilt that will be eventually underneath once you bring your mitered corner down. I hold it in place with my thumb nail and then pin. Sometimes I have to massage the miter a little but not much.
To prepare my quilt for the binding I stitch around the edge very closely before I trim the excess backing and batting off. This stitching works much like stay stitching in garment construction in that it not only holds the layers together but also keeps them from stretching.
The challenge I have in sewing the binding on once it is turned (rather than hand stitching it) is getting the stitching to appear even along the edge of the binding. I think if I carefully turned my binding and glued it down evenly then when I turned the quilt over to stitch in the ditch on the front it would evenly catch the binding in the back. I'm sure I couldn't accomplish that with just pins.
Sharon Schamburg has a video about bindings where she glues the binding on first and then sews, then turns and glues again. You might want to try this technique to get your corners better.