Fill every void with ice — and prechill everything.

Prechill the cooler and everything going in it, then pack as tight as possible, filling every air gap with ice.

Air pockets are the enemy. They let warm air circulate, which melts ice faster than you can drink that first lukewarm can. The goal is thermal mass: the more cold stuff touching other cold stuff, the longer it stays cold.

Start with a cold cooler (stash it in the fridge overnight or throw a bag of ice in for an hour before packing). Chill your drinks and food first too — warm bottles dumped into a cooler with ice will melt it in an afternoon. Then pack dense: cans snug against each other, bottles nestled in, and pour loose ice into every crack. If you have room left over, add another bag of ice instead of leaving it empty.

For long trips, use block ice or frozen water jugs. They melt slower than cubes. And keep the cooler out of direct sun, open it fast, and don’t drain the water until you’re done — cold water still insulates.

Ice is cheap. Air is not.