One thing to remember about mitering corners is that the distance from the edge where you stop sewing needs to be the *same* as the seam allowance you are using. I remember the first time I sewed a binding on with a 3/8" seam; believe me, you want to stop sewing 3/8" from the edge -- not 1/4"! I had to completely remove the binding because the corners wouldn't turn.
Also, I have found that stopping just one stitch early helps my corners turn out better. If sewing a 3/8" seam, I stop 3/8" + one stitch early.
As I turn the miters, I typically will trim off some of the batting from the corner tip. I usually try to trim only the quilt tip, not any of the binding material. This too helps me get a better corner miter.
Edit: As others have mentioned, *not* ironing the binding in half seems to help too. Once I started doing that, I noticed that my bindings roll easier.
When sewing on binding, I have the binding on top and keep it slightly taut as I sew. This ensures that I do not sew on more binding than there is quilt!