Of the flats I've experiences over three years and 7000 miles, two were failures at the valve stem. I spent some time pondering this problem including questioning the local bike shop. What I discovered after much pondering and probing was that the rim itself, after boring out the aluminum at the plant, was left with a sharp and somewhat ragged edge at the valve hole. This is known as a manufacturing flaw. My solution was to peel back the rim tape a few inches and chamfer the valve hole with a tiny rat-tail file and 150 grit sand paper. This alone should have resolved the problem but in addition I also cut out a small piece of rubber from an old tube, about 25 x 25 mm, with a very small hole and pressed it on over the valve for extra protection of the tube. So far, in the last 2000 to 3000 miles, no flats of any sort.
As extra, extra precaution, I also inspect the tires every 7 to 10 days or so and pick, out of a deflated tire, any glass or debris I find embedded in the rubber. I frequently find potentially flat producing junk in the tires which is not surprising as there is all sorts of trash on our roads.