The best way to keep your lawn as weed free as possible is to start early. Now, depending on where you live, some can start earlier than others. Just be sure that the ground is not frozen.
Corn Gluten is one of the best weed preventers on the market for numerous reasons, the first being that it is a 100% natural and organic product. It is safe for using around pets, children, and wildlife. It is also highly effective. In a multi-year study on a crabgrass infected lawn that was treated twice per year, Iowa State University reported a 58% reduction in weeds the first year, 85% the second year, and 91% the third year. The more consistently you use Corn Gluten, the more effective it is. As with any organic product the results are not instant. Chemicals give instant results along with a lot of consequences.
You will want to use it in early spring, and again in early fall. What you are doing is catching the weeds before they hatch, or germinate. Keeping this in mind, do not use the Corn Gluten if you are seeding or over seeding your lawn. It will prevent your grass seed from germinating as well. You will apply about 20 pounds per 100 square feet in a drop spreader.