There are cool-weather crops that, if you start 'em now, will produce right up to the first frost and beyond. Broccoli, cauliflower, Brussels sprouts, lettuce, radishes, carrots, collards, garlic, kale, lettuce, mustard, swiss chard, spinach and turnips are all 'frost tolerant' crops. Some, like brussel sprouts and carrots actually taste better after short exposure to frost.
Prepare your soil as you do in the spring. Clean out all of the warm weather crops and add organic matter and work into the soil. You have to keep the soil moist, especially if we don't get a lot of rain (not a problem so far, but you never know). Plant seeds deeper than in the spring so they will be in a moister and cooler layer of the soil. To increase seed germination, water the planting area with a fine mist 30 minutes before you plant. This lowers the soil temperature and creates the conditions that cool-season crops prefer. Stress is also a big factor in August. To aid in moisture retention, mulch after seeding and transplanting. Don't cover the plants, of course, but if you put hay or grass clippings all around them you'll help the soil retain moisture in a big way.
Our first frost is usually around the last day of September. The first hard frost is a month later, October 30. Lots of vegetables can withstand several frosts. Remember, they'll grow slower because there's less and less sunlight every day. When you do hear there's going to be a frost, you can cover stuff up overnight, with a brown paper bag (and boy does that look silly), or sheets, just to prevent the dew from getting on the plant. Remember to uncover the next morning.
Here's a list of how long things take from seeding to harvest, and just how tolerant they are to cold, and some suggestions for varieties that are good for fall planting:
Spinach: about 60 days, tolerant of frost but not of prolonged freezing...Bloomsdale Long Standing, Winter Bloomsdale.
Lettuce: 55-60 days, tolerant of some frost....Black Seeded Simpson or Oak Leaf
Broccoli: about 75 days, tolerant of frost and maybe down to 29 degrees....Waltham, Sweet Valiant
Cauliflower: same as Broccoli...Snow Crown, Violet Queen (if you like purple)
Carrots: 75 days, they're fine until the actual ground freezes
Beets: 65-70 days, they're more tolerant of freezing but not for too long.
Brussel Sprouts: 80 or 90 days, so it's probably too late...but they're DELICIOUS after a frost.
String beans and Lima Beans will die in frost, but if we get lucky and there aren't any until, say, mid October, you could plant now (get out there RIGHT now) and still have beans in the fall. I have tried fall peas but I think the day length affects them, they hardly ever grow to adulthood..
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Broccoli, cauliflower, Brussels sprouts, lettuce, radishes, carrots, collards, garlic, kale, lettuce, mustard, swiss chard, spinach and turnips are all 'frost tolerant' crops. Some, like brussel sprouts and carrots actually taste better after short exposure to frost. 
Stress is also a big factor in August. To aid in moisture retention, mulch after seeding and transplanting. Don't cover the plants, of course, but if you put hay or grass clippings all around them you'll help the soil retain moisture in a big way.
