I know that one of the things I sometimes have trouble with is keeping a clear image of a character in my mind. Did that guy have a goatee, or was it a full beard? Didn't I just mention how that guy's nose was a bit hawkish?
Othertimes, it helps to have the character looking at you in order to give them a name. But how could you do this without having access to a professional police sketch artist? Ah-ha, how about a computerized sketch artist? We, here you go - a flash version of facial composite software. http://flashface.ctapt.de/