Introduction: LiveStrong Bicycle Flashlight
Simple, really. Take a Lance Armstrong "LiveStrong" bracelet, and stretch it around a flashlight like the picture shown. If you don't have a Lance bracelet, then use any similar rubber bracelets. In the picture shown, I am using a 1-Watt Luxeon flashlight, commonly found for about $10US, nowadays.
I don't think this would endure the rigors of mountain biking, but it certainly would work for road bikes. If you are worried about it possibly falling out, then attach a lanyard (strap) to the end of it, and loop that around your handlebar, then pull it through, before mounting the light. Then if it comes loose, it will simply be dangling from your handlebars.
By the way, in my opinion, Lance Armstrong is the finest cyclist in the world.