Don't iron it in half (lengthwise) before you sew it on.
I'm serious. It sounds like too little, but it really works.
I always square and trim my quilt before stitching the binding on. When applying the binding, I align the raw edges of the binding with the raw edge of the quilt and stitch 1/4" in. Wrap the binding around, and since I didn't iron it first, the fabric is free to adjust itself and wrap nice and snug. Then stitch to the back. I use either a 2" or 2.25" binding. My bindings are show quality and I have a happy base of customers who pay me to do their bindings.