Just use an o-ring. This o-ring is one of the large ones that came with my Garmin Edge 500. You can also pick these up at pretty much every hardware store for less than a dollar. A good idea is to cut up an old tube to place between the flashlight and the handlebars as well. This actually works really good. That is one of those $3 Miraclebeam flashlights from Fry's Electronics. The brightest thing you can buy for $3.
Good lighting for $12. Get 4 of those flashlights. 2 to the bars and 2 to the helmet using the same exact sized o-rings. I just tested this out the other night and I was surprised just how well it lit up the area in front of me. You will get lighting for $15 total here (lights, o-rings, and taxes) that is equivalent to the $50 to $100 lights you see in the store. More flood like lighting than beam, but you can also get beam type lights for about $3-$5 at Fry's you can use on your helmet instead.