I've had good luck with those clamps, but I fold the edge of the tarp around a small stick (about half the size of a pencil, or so), then insert the folded edge into the clamp so that the tarp-wrapped stick gets into the round area behind the jaws, and the jaws then clamp onto a double layer of tarp. I use these to hook bungee cords to a weight, or sometimes a pole or 2x4 running the length of the pile. This alows for a little give when it's windy, and keeps the clamp from pulling loose.
I saw a vid the other day that used to pieces of pvc tube....sized so that one would fit into the other. Put smaller diameter pupe into larger. Cut to about 3ish inches. Take the larger piece and cut through one side lengthwise using a hacksaw. Then roll the tarp corner around the smaller size pipe, 'clamp' the larger piece over the smaller one and tarp. Then you can run a cord through the inside piece to hold down a corner. It was pretty slick. Think I'm explaining it right...
