(Editor's Note: This article was originally published on September 28, 2007. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
Cool season gardening really begins in the heat of summer, in July and August, when its time to start seeds for the fall/winter garden. The best introduction to selecting seeds to plant for fall that I have seen is Rob Johnston, Jr.'s newsletter for Johnnys Selected Seeds. Cool season crops are essentially salad and cooking greens, especially all of the brassicas (cabbage family); root vegetables including carrots, beets, onions, shallots, leeks, and garlic; and oriental stir-fry vegetables. The time to start fall seed crops is determined, first, by finding the first frost date for your area. This should be available from your local extention agent. Then, find the number of days from seed-to-harvest on the seed packet. Add a few weeks for slow growth if the crop will be growing in cooler weather. 
Cool season vegetables, from left to right: kale 'Dwarf Blue Curled Vates'; common garlic; swiss chard 'Bright Lights'; chicory 'Catalogna Special'; carrot 'Purple Haze'; mache or corn salad; radish 'French Breakfast'; and bok choy or Chinese cabbage.
Here is a thread in the Vegetable Garden Forum: Winter Vegetable Gardens from November 2005 through February 2006. Farmerdill and the gang discuss their own winter vegetable gardens.
Don't forget to plant some flowers for fall bouquets. This is the list from Johnny's Selected Seeds.  To this list I would add for southern gardens: dahlias, chrysanthemums, and hardy cyclamen, and the ubiquitous violas and pansies. The photo on the left is Viola 'Helen Mount' - a named cultivar of johnny jump-up type viola.
To raise a successful fall/winter crop you need excellent soil and perfect drainage. Many people use raised beds. Here is Tabasco's thread from the Perennials Forum on building "lasagna beds". An early proponent of intensive gardening in raised beds, Jon Jevons,  recommended that a 5 foot by 20 foot t space is adequate to provide food for one person. The beds should be located for maximum exposure to winter sun. But, if seeds are started in the summer heat, some type of shade should be provided for the young plants. Raised beds allow easier installation of season extending devices, such as shade cloth, double plastic 6mm film stretched over a PVC hoop frame, row covers, and Remay winter blankets. Shade cloth, or 6 mm plastic can be attached to PVC hoops spanning the beds, anchored in 1-1/2 inch pipes sunk on either side, and secured with clips. Lengths of PVC pipe, one size larger than the size of the PVC used for hoops, slit down one side can be used for clips to secure the fabrics to the hoops.
If you are using the same space you used for a spring crop, keep in mind that the same types of crops should not be planted continusouly in the same soil. If you planted cabbage at one end of your bed last year, don't plant any type of brassica crop in that space the following year. Keep a journal so you know what sections of your beds were used for what type of crop in each season. You will need to plan for crop rotation over the garden space. It is a good plan to reserve small sections for cover crops if the soil needs to be renewed. 
One of the most successful experiments in winter gardening is Eliot Coleman and Barbara Damrosch's Four Seasons Farm in Maine. Using Remay row covers, cold frames, and portable plastic covered hoop houses, together with a judicious selection of cold hardy plants, Eliot Coleman was able to keep his gardens in production throughout the inhospitable Maine winter. 
My favorite resource for winter gardening, or for any gardening question, is Bernard McMahon's An American Gardener, first published in 1802.  This is a calendar of gardening and detailed description of tasks to be done for every month of the year. His detailed descriptions of how to build hot beds include using tanbark, available from tanning yards, or oak leaves, or fresh horse manure. These materials will produce enough heat as they decompose, to bring a crop through winter freezing temperatures. To build a hot bed in January, I will need to dig my 5 foot by 20 foot bed 3 feet deep. Then I will need to fill this area with fresh steamy horse manure. Lets see: for the 5 by 20 foot by 3 foot area I will need 300 cubic feett of fresh horse manure . . .. I don't have a horse, but I do have a rabbit.
 Johnnys Selected Seeds newsletter, August 2007
 Jon Jevons. How to Grow More Vegetables Than You Ever Thought Possible on Less Land Than You Can Imagine. July 1985. Paperback.
 Bernard McMahon's An American Gardener: Adapted to the Climate and Seasons of the United States, Containing a Complete Account of All the Work Necessary to be Done in the Kitchen-Garden, Fruit-Garden, Flower-Garden, Orchard, Pleasure-Ground, Vineyard, Nursery, Greenhouse, Out-House, and Forcing-Frames, For Every Month in the Year; with Practical Directions and A Copious Index. First Published in 1802. Now in Paperback.