In order to get nice corners, make sure to stop sewing slightly before the 45 degree line.
If you cross that line, there goes your nice corner.
I use the Perfect Binding Miter to mark that 45 degree line and it really helps.
http://www.connectingthreads.com/too...r__D81892.html
Edit: Also, make sure to square up your quilt before you sew the binding on.