A palm held flashlight is vital when in the field. From making a night-time expeditions to latrine to reading during a blackout, a flashlight is essential.
An important trait of any good flashlight. You don't need a 200,000 candlepower spotlight, capable of illuminating entire villages... Instead the flashlight should be slightly smaller than your palm. A small flashlight can be strung onto a keychain with a lanyard. The lanyard allows your flashlight to be hung as an overhead light when needed, but more importantly for forgetful blokes like myself; it makes the flashlight nearly impossible to misplace. Also, a palm sized flashlight permits the light to be grasped in your teeth, allowing hands-free directional illumination. My own flashlight is wrapped in ¼ inch of electrical tape, which provides a more comfortable tooth grip.
Get LED; an LED light has longer battery life, less weight, and similar candlepower. I normally carry 6 backup batteries for my LED. This affords me over 1,000 hours of light and takes up as much packing space as one double A battery.
Your flashlight should be bombproof. It is going to be crushed, thrown, dropped, hit, shook, and lost. This considered, never buy a plastic flashlight. Plastic construction might be fine for the emergency box in your house, but not on a volunteer trip. I tested my light by dropping it eight floors onto concrete, but even I consider that excessive.
Here is the good news, the flashlight I use cost me six dollars and has lasted for eight years. No need to scour the overpriced para-military stores for a light; Sport Mart, Wal-Mart, REI, and eBay all have them. If you want to go a little more extreme you could find a waterproof LED light but that would cost a little more.
So there you have it. Hope it helped.