Remember that the coldest part of a cooler is on the bottom, because cold air falls and heat rises. Pack the most perishable foods, like meats and dairy, on the bottom.
Scatter ice packs throughout the cooler. You can use the ice packs sold in stores specifically for this purpose, or make your own by freezing ice in a clean, rinsed water jug or bottle. Placing ice in a heavy-duty resealable freezer bag is also a good choice. When the ice melts, you can use the resulting cold water for drinking.
Store the cooler in the shade, not inside the hot car.
And don't count on the cooler to cool foods which have been warmed. After foods have been out of the cooler for two hours throw them away.
