I use clamps from my workshop to hold down my long rulers on my cutting table to cut batting and trim completed quilts. They're very sturdy as they're metal and have rubber covers on the portion that touches the fabric. It takes both my hands at times to open them to clamp over so that tells you just how strong they are. They're called spring clamps and this page on Harbor Freight shows the 2 types I have.
http://www.harborfreight.com/catalog.../?p=2&q=clamps I use the ones that are all metal in my sewing room.