Lip lights are worth their weight in gold, if you need one. Various kinds, the LEDs don't burn out, you don't need a hand to operate, they point where your head points, NVG compatible, not a lot of spurious light dazzling out the cockpit, etc.
Depends upon what kind of flying you do, really, and how your machine is equipped. Someone with a NVG-compatible EMS or military cockpit will normally use a NVG-compatible green lip light. Someone in the R-22 with a Mag-Lite duct-taped to the console maybe uses candles, for all I know, which begs the question why are you flying at night in a piston single anyway unless you have a death wish.
If your cockpit is properly lit, you really need only a minimal amount of personal lighting and too much can be a hazard. If it's not, well, then you probably need more. I haven't used a conventional flash of any kind in a cockpit in years... too much light. If you're using your light to illuminate gauges, a lip light will work, but a cockpit mounted and dimmable flood might be better. If you only use it to write with, either a finger light or a lighted kneeboard.
http://www.s-lite.com/