Crop Rotations - certain groups of weeds are associated with specific crop rotations. Changing the crops can reduce or eliminate repeat, persistent weeds associated with those specific crops.
Plant Competition - One of the cheapest and most useful general weed controls available to all farmers. Survival of the fittest.
Crop Establishment - Generally the first plants to germinate and emerge in an area tend to exclude all others, so work toward a vigorous, dense crop.