I learned this from another website and it worked pretty well.  All you have to do is find a rubberband thats about 1cm wide and take a razor blade and cut it to a really small circle.  Then you poke a hole in the middle of it with a pin.  Then take the little circle and push it around the fill valve on the bottom of the clip, that part will be hard and you might have to use some pins or something to push it around.  After you get it around try it out and see if it works.  If it doesen't work you may have cut the rubberband to small or just messed it up when you pushed it around the valve, or just make sure your holding the the can and the clip straight when your filling.