A cooler is a container for cold, not a fridge
Coolers do not make anything cold. They only slow the rate at which cold escapes. That single fact drives every packing decision you make: start cold, keep it dense, and open it as rarely as possible. Done properly, an ordinary soft cooler will comfortably outperform an expensive hard one that has been thrown together at the last minute. The technique is worth more than the price tag, and all of it is free.
Pre-chill the cooler and everything going into it
A warm cooler spends its first hour cooling its own walls instead of your food, and that hour comes straight off the end of your day. The night before, put a sacrificial bag of ice inside and leave it somewhere cool rather than in a sunny garage or a hot car. Chill the drinks in the fridge first as well — dropping room-temperature cans onto ice wastes a large share of your cooling capacity before you have even left the house. If you are freezing food such as cooked meat or a batch of sauce, do it two days ahead so it is frozen solid rather than merely cold, because a solid block acts as an extra ice pack for the first few hours.
Layer from the bottom: frozen, then cold, then fragile
Cold air sinks, so the bottom of the cooler is the coldest zone and should hold whatever must stay coldest: raw meat, dairy, anything you would not leave out of a fridge. Start with frozen gel packs or a frozen bottle of water flat on the base, then dense chilled items, then a middle layer of drinks, then soft things like fruit, salads and anything that would bruise on top. Finish with a second layer of ice packs laid across the surface. That top layer matters more than people expect, because it is what stops warm air from settling down onto the food every single time the lid is lifted.
Block ice lasts, cubed ice cools, and meltwater helps
Cubed ice chills things quickly because it makes contact with more surface area, but it also melts quickly for the same reason. Frozen bottles and solid gel packs melt far more slowly and double as cold drinking water later in the day. The practical answer is to use both: solid packs for endurance, plus a little cubed ice poured into the crevices so there is contact rather than air. And resist the urge to drain the meltwater as the day goes on. Cold water conducts heat away far better than air does, so provided your food is in sealed containers, that slush at the bottom is actively working in your favour.
Eliminate air, and split the load if you can
Air is the enemy of a cooler. Every gap you leave is a pocket that has to be cooled again each time the lid comes off. Fill whatever space remains with a rolled towel, spare drinks or crumpled newspaper. Better still, use two coolers: drinks get opened constantly while food gets opened perhaps three times, so separating them lets the food cooler stay shut and cold all afternoon while the drinks cooler absorbs the traffic. If you only have one, at least keep the drinks in the top layer so nobody has to dig past the salad to reach a can, and decide in advance who is on lid duty.
Keep it shaded, and know what good looks like
Shade is free insulation. Put the cooler under the table, behind the blanket line, or under a light-coloured towel — direct sun on a dark lid adds a surprising amount of heat to the surface. On sand, half-burying the cooler keeps it against cool ground rather than baking on top of it. When you leave, load it into the car last so it spends the minimum time in a hot boot. A well-packed cooler is heavy, silent when you shake it, cold at the top when you open it, and still holding ice at sunset. If yours is half-empty, rattling and lukewarm by mid-afternoon, the cooler itself is almost never the problem.