Use an ice source, and keep the grill in the shade.
Pack drinks in a different cooler than the food, to keep each cooler colder longer.
Marinate foods in the refrigerator, and don't consume uncooked marinade. If precooking in the microwave or oven, cook the food thoroughly so all internal bacteria are killed. Partial cooking is fine if the food will go immediately on the grill to finish it off. Take out only the amount food you can cook at one time. Do not put cooked food on a plate that held raw food.