Here is a Youtube video that shows one brand of side tensioning clamps:
http://www.youtube.com/watch?v=JfNXIT2uZ3w
My Hinterberg frame came with side tensioning clamps, but they are not very good. I need to either purchase side tensioning clamps such as those in the video or experiment to make my own. Some people use homemade combinations such as bungie cords and curtain rods to make side tensioning clamps. Clamps tend to interfere with your machine when you get to the side -- your machine starts bumping into them long before the needle gets to the edge -- and that is the problem I am working on now. I also purchased a long tape measure that will go over my quilts so I can make sure that the width stays the same all through the quilt.