i have a friend who always had trouble with mitered corners on her bindings- her answer was to round her corners---she uses a salad plate and cuts each corner into a curve- then using bias binding just binds- perfect every time-
i don't have problems with the miters so i don't tend to use her technique- but she makes absolutely fabulous quilts! all with rounded corners.
but just make a little dot at the 1/4" spot at the end of each side 1/4" from the end of the quilt side- in both directions- stop sewing right on that dot. take the quilt out- fold the binding up straight- fold it back down- line up the straight edge again and start sewing on the 1/4" dot and continue down to the next end. when you fold the binding to the back as you reach a corner fold the side you are approaching down- then fold the side you are on over it- forming your miter- then stitch to it- stitch the miter closed- and continue down the next side.