The first trick is to secure the strap by running a small piece of tape or a zip tie through a vent hole to hold the strap in place. That's what my son used during our bike trip last May to Sacramento.
Because the Energizer Trailfinder light can flip up, I was also able to run the strap around the back of the helmet and put the light flat on top, like so:
At the rear of the helmet, the strap runs near the back retention straps and doesn't seem to interfere with the helmet fit.
Another helpful feature of the Energizer Trailfinder: It has a pair of red LEDs in addition to the white LEDs!
Something like this is handy if you're caught out later than expected without bike lights and all of the local bike shops are closed. These kinds of lights are available at any outdoor retailer and many drugstores and mass market retailers.
This is mostly a hiking / camping light. The Trailfinder will leave this weekend for a 10 day backpacking trip through the Sangre de Cristo Range so we'll see how it holds up under harsher conditions. For urban bicycling, the light stays on and the white and red lights are both reasonably bright. Unlike most bike lights, the Trailfinder only has "steady on" modes -- there are no flash or blink modes for this light.
If you use hiking headlights for your night time bicycling, I'd love to learn your tips and tricks.