www.fpl.fs.fed.us/documnts/techline/crawl-space-ventilation.pdf gives some good ventilation info. You didn't say what kind of foundation you have, chainwall, poured concrete, pier and beam? As far as ground cover, a lot of people use 6mil poly but a cross laminated vapor barrier is better, though quite a bit more expensive (and worth it, in my mind). Overlap the barrier and seal it with vinyl tape. 3m makes several for the purpose, I've used the 6mil gray. If you have a walled foundation, run the barrier about 6" up the sides and seal with adhesive.