Today I got a tip in a writing workshop that I later realized can easily be applied as a GM tool. The tip was to, when describing a character in prose, give three concrete details with which the reader can anchor their conception of the character. Strictly speaking, these don't even have to be physical descriptions of the character, and might be an action the character is performing at the moment of the description, such as smoking a cigar, tapping her foot, anything that you feel expands the character's implied persona in the direction you want.

The reason this dovetails so well with roleplaying is that it's quick, easy, and to the point; in other words, all the things a GM tool needs to be. It provides an handy framework in which to place the details you find most essential to an NPC's being while providing limits so you don't accidentally put your players into detail overload.

That said, the rule is not set in stone. We all accept that not all NPCs are created equal, so I offer this corollary to the above rule: Generic NPCs get a two point description, and NPCs of great importance (BBEGs, for example) get four points.

Any other NPC description tips/techniques out there? Sound off in the comments.