Brew Your Own's latest issue has an excellent article on this topic. They say to wait for autumn nights when the temperature drops to about 40F. Brew your beer as normal, but leave the lid off the boil kettle at the end and let the lot wort cool for about 12 hours. Then rack into the carboy or bucket, and pop on the airlock. You could also set out several small pots of hot wort in various locations to use as "starters." If you're a techie, you can also leave out petri dishes of agar to catch the wild microflora. One very important thing: you can get some nasty, harmful bugs (Salmonella, E. coli). Evaluate the fermenting wort by smell only for the first month. By then, the pH of the fermentation will have dropped enough to kill off any potential nasties. Then you can taste. I plan to try some open air fermentations very soon. I highly recommend a trip to Allagash Brewing in Maine. They've pioneered the open air fermentation with their "Coolship," which looks like a small stainless steel, open air swimming pool.