Before I got a machine quilting frame (and after pinning the layers together) I would use bicycle clips (something like this
http://store.quilting-warehouse.com/notions-clips.html ) and roll each side towards the middle, but leaving enough space to quilt down the middle. When I finished one row, I would unroll one side a little bit and take up the difference on the other side so that I could quilt on the next row. I hope this helps.