My best results for writers have come from craigslist ads.  Also my worst results.  One of the absolute best quality producers was located 3000 miles away from the subject (local museums) but interviewed their staff via phone and knew how to get top-notch photos released.

>very simple criteria to give them a guide.

I keep a spreadsheet of topics and a list of "keywords to think about" as well as a list of things to avoid. I also give them a demographic profile of the user that's most likely to be using the site --caucasian, female, married, 2 kids, dog, only vaguely familiar with the topic, etc.

I think the best way to get good writing is to specify exactly what you want.  Very often, I find that people get crap because their requirements are very vague.  RC is dead on when he tells the writer about WHO he needs to write for.

And the thing is, unless you've had to write for someone else, you DON'T know what to tell someone writing for you.

I may write something that is really good content-wise, but has the wrong voice for the audience.  I point to my template that I always use and have had very very good luck with located here:

http://th3core.com/talk/web-development/how-i-find-good-content-writers/