I have a set of wheels where this is prone to happen. My guess is that there's a sharp edge around the valve hole some place but I can't locate it or apparently do much about it. It's a double walled rim and I can't get a good look. So I took one of the ruined tubes and cut 2 sections out of it, - about 1.5 inches in length. In each one I poked a hole large enough for a valve stem to slide through. The hole is through both sides of the tube so I have essentially two layers.
I then took each of these sections and slid them over the valve stems on the tubes I intended to mount on the wheels. The idea is that they will protect the base of the valve stem from any sharp edges. One thing I no longer do is crank down that nut that comes with presta valves. I screw it down enough so that I can get a pump head on it, but not a ton more. Doing this has pretty much eliminated the problem.
Last edited by tjspiel; 05-30-11 at 11:24 PM.