Select the 200 finest corn plants from your current crop. Look at the plant's resistance to adversity (weeds, dry or poor soil), its taste and its yield. Reserve the ears from these 200 plants for seed collection.
Run each of the ears through the corn shucker. Discard the husk, cob and greens and lay the seeds out for sorting. Discard any seeds that are withered or broken.
Dry your corn seeds for at least 2 weeks in indirect sunlight before packaging and labeling them in paper bags. (If mice or other animals are a problem, seal the paper containers inside a plastic storage bin and out of the way of animal-trafficked areas.)
Identify what traits you're looking for in your crop. Consider things like average yield per acre, soil type on your acreage, fertilizers, weather patterns and the intended usage for the corn--whether it is to be livestock feed, sold on the export market or trucked to the farmer's market for human consumption.
Talk to local farmers or the local seed bank, if any. Ask if they grow open-pollinated corn on their acreage, and what varieties have worked for them. Be sure to ask about any problems endemic to your area (wildlife, soil conditions, etc.) and what methods of fertilization they use.
Visit heirloom and organic seed stockists such as Victory Seeds. Look for open-pollinated corn varieties that display some or all of the traits you're looking for. Before investing in enough seed to plant your acreage, look for reviews online at Seeds of Change and other websites. When you've decided on a variety, purchase your seeds.