1. Proper PH
The clovers, brassicas and chickories that make up the majority of food plot mixes grow best in soils with a PH near 7 (neutral). Your local county Agricultural extension agency will often test samples for free or companies such as Antler King make inexpensive kits that allow you to test on the spot. Once you know the PH of your soil apply lime in approprate amounts to bring your soil to the recommended range. Your extension agent can recommend the proper amount or refer to the chart in your Antler King test kit.
2. Proper seedbed preperation.
The soil should be plowed and tilled enough to break up the root systems of the previously established plants. Spraying is not recommended as seedbed preperation exposes dormant weed seeds which are unaffected by the spray and can quikly germinate. Once seedbed is prepared broadcast plant your seed but either do not cover at all or cover by dragging a fence section etc over the soil to just barely mix in the seeds.
3. Proper fertilization.
Fertilizers are labeled with a three number label such as 30-30-30. The first number is Nitrogen, the second Phosphorus, the third Potassium. (N,P,K) To establish your food plot use a fertilizer high in Potassium and low in nitrogen. The desirable plants are Potassium lovers and the grasses and other weeds are usually Nitrogen lovers. When your planted seeds germinate a fertilizer such as 0-0-60 will boost their growth allowing them to overshadow and choke out the weeds that germinate along side them.