I struggled a bit trying to figure out how to mount my Fenix L2D flashlight to my helmet, and thought I'd share a solution I'm happy with. The helmet is a Bell Furio, and I'm using a Zefal Doodad Plus as my mounting "medium." It seems to hold the flashlight tightly, and is easy enough to attach and remove quickly.
While I'm at it I might as well share photos of my handlebar light setup. I'm using a DiNotte 200L mounted on a Nitto Lamp Holder, and strapping the battery pack to my stem using the included velcro strap. The Nitto is rock solid and I'd highly recommend it.
Perhaps this post will shed some light on a useful headlight setup and inspire other ideas I hadn't thought of.