Headlamps! Everyone has one or wants one. What's good what's bad? Should I even have one?
Anyway, When it comes to light, you wanna find something that has the highest lumen count, using the lightest batteries that'll last for as long as you'll need to be on the trail. Each and every one of those attributes fight against each other.
So what does that all mean? Well the higher the lumen count, the brighter your light will be. If the count is really high, then it either needs lots of batteries, or goes through batteries, like me through french fries and beer!
If your light requires lots of batteries or big batteries it probably is to unwieldy to use as a trail running light. On the other hand if it goes through it's batteries in a couple hours that's not good either cause you're spending more time changing your batteries than you are actually running.
So what kinda attributes are you looking for then in a headlamp? Well I'd say anything under 50 lumens is not enough. And if you're going for the 70+ lumen count you should be looking at lights that use Lithium batteries. Laurie "Pixie Hawk" uses the "Saint Minimus from SureFire":http://www.rei.com/product/796823
Though something else to consider is if you can get a steady hand while you run you can use a handheld light. Like me! you can get away with using a light that uses considerably less lumens, I used a "Princeton Tec":http://www.brightguy.com/products/Princeton_Tec_Impact_2_LED_Flashlight.php for the Leadville 100 last year. It's not the brightest light in the world, but they way it focuses it's light is pretty handy. Also when you use a handheld light it shines the light from a lower angle, meaning you get a better idea of the contours of the ground and thus your depth perception isn't all wonky.
I do have a handheld (it was a gift from my mom after I finished LT100) that produces much more light, and it's a beauty to have on the trail at night. I feel like I'm holding a bit of the sun in my hand, but I don't know what it's called. It's a no name light, with the LT100 logo on it and it goes through batteries like mad. My Princeton Tec lasts forever! I think I replace the batteries in that thing like once a month or so.
So there ya go, I hope that gives you some ideas of what to look for in lights.
Also when looking for headlamps, you want something that's not gonna give a lot of bounce on your head. I highly recommend trying some on, to get an idea of how they feel.
Also keep in mind, when you run at night when it's cold, your breath will be illuminated by your headlamp and block your vision of the trail. The biggest reason I switched to a handheld. There are plenty of people who run with both as well, which is a good idea too, cause hen you can get away with cheaper headlamp (less lumens) and cheaper handheld, then you get the benefit of having light where your face is pointed, as well as having light on the trail at a lower angle giving you better depth perception.
Anyway, I hope that gives you some ideas on what to look for.
Note: Gary "Story Hawk" uses this "headlamp":http://www.rei.com/product/768981 and if I were to buy a headlamp I would probably buy that one, or the one Laurie uses. Laurie's requires special batteries, which kinda sucks, but it makes for a pretty awesome headlamp, with great power, great battery life, and light weight. You just gotta make sure you buy the batteries in bulk, and have some on you at all times, cause it's almost guaranteed any race you do, NO ONE will have spare batteries for you.