beat this:
https://www.youtube.com/watch?v=FZSg42ZbGbw

Clever!

For the less do-it-yourselfer, or the guy who has no reflectors --

https://www.dealextreme.com/details.dx/sku.8274 $1.88 bike mount

Or if you don't like these flashlights, these lights are a good deal brighter and only a tiny bit more expensive --

https://www.dealextreme.com/details.dx/sku.47706 $7.60 3W 60-Lumen 2-Mode Front White Light with Mount
https://www.dealextreme.com/details.dx/sku.18307 $7.43 1W 2-Mode 50-Lumen LED Bike Light with Mount

Easy to beat that. Take a hunk of wood, about 1.5 inches square by a few inches long.. Drill two holes, one the diameter of your handlebar, the other coming through at a right angle and so that there will be half an inch or so between the two holes. Cut the wood straight through the center of both holes. The thing you just cut out of the middle is basically a lockblock. Secure your light to the bar with that and a few rubberbands.

Net cost basically nothing. I used a block like that for a few months a while back.

I use about 6" of inner tube, and cut 2 slits (halfway through the tube) transverse to the tube about 2" apart. Wrap it around the bars and use two cable ties inside the tube to hold it in place. The flashlights slip through the slits. It's one size fits all and surprisingly stable.

If you have to tell someone how to lash a light to a bar -then they might as well just get "hit" instead.......

For a couple of years I used a piece of inner tube, cut to suitable length and tied to a loop. Basically a big rubber band.

all great ideas, keep em coming!!

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.

my reflector setup was FREE and the flashlight was $2.99
talk about LOW DOUGH!

I have one light that is held on by zip ties