Way back in 1992, Dragon Magazine #184 featured an article by C.M. Cline entitled “The 7-Sentence NPC,” and it’s by far one of the best GMing tools I’ve ever used. I mentioned it in 2005’s Vibrant NPCs  here on TT, but I thought it deserved a post of its own.
The basic concept is that you can use a seven-sentence template to describe nearly any NPC — and although it appeared in Dragon, it’s not limited to fantasy characters by any means. Each sentence covers a specific aspect of the NPC, from appearance to memorable quirks, and the end product is both remarkably detailed and simple enough to reference on the fly.
I’ve used this technique for years, and found that it gives me everything I need (except stats, of course) for minor to significant NPCs, and lays a good foundation for major NPCs, who usually need a bit more expansion. And best of all from a prep standpoint, it only takes a few minutes to write up an NPC this way.
So what’s the catch? It’s from 15 years ago, and unfortunately it’s not available in PDF. On the upside, though, it is available in a few different forms: Paizo sells issue #184 for $15 , which, while expensive, I’d say is worth it just for this article; it’s included in the recent Dragon Compendium, Volume 1 hardcover, which you can probably find online for less than its $40 cover price; and of course it’s in the Dragon Magazine Archive CD set, which is sadly long out of print.
The good news is that the seven sentence topics themselves can be found online: Critical Hit  member Titania posted a summary of the article . It’s missing the nuances and examples for obvious legal reasons, but there’s enough there to give you a good idea of what the SSNPC concept is all about.
My apologies for talking up a GMing resource that’s neither cheap nor easy to find, but I figured that highlighting this article for GMs who haven’t heard of it before outweighed the potential frustration — it really is that good.