Freeze all your food first. The colder everything is to start, the longer it will stay cool. Plan your meals to eat the most perishable foods first.

If you carry drinks, freeze them too. When talking about frozen liquids, keep in mind higher surface area means faster cooling and shorter lasting cubes. So, a frozen liter bottle of water will last a long time, but not cool quickly. If possible, use a couple different sized bottles. That way, they will melt at different rates: day one, the small bottles melt; day four, the big ones will have melted.

A bigger cooler requires more ice to keep the same stuff cold. Use the smallest cooler you can. If you can, find a good collapsible cooler and smoosh it as you use food.

Get reusable ice cubes; liquor stores carry them. Get the water filled ones since they do fail occasionally. As you fill the cooler, put a layer every 2-3 inches. More importantly, fill all the spaces with the cubes. Pack them tight, leave no voids. Voids are just room temperature air that needs to be chilled.