A cooler packed with ice is the easiest way to keep frozen food frozen, or at least cold enough to be safe to eat later. When the temperature of food rises above 40 degrees Fahrenheit, it's good for about two hours before bacteria starts to increase to unsafe levels. You can put frozen food transported for a few hours in a well-packed cooler back into the freezer when you arrive at your destination, and that same cooler can extend the safe life of your food if you're camping.
Pick Your Cooler
Inexpensive foam chests work well to keep food cold, particularly for shorter trips. For an extended road trip, or if you're going car camping, a rigid cooler made from fiberglass or plastic is a sturdier option. Like Goldilocks, you need something that is just the right fit. If you plan to take a lot of food, two small or medium coolers might be easier to carry than one large one, since you need to pack the coolers full to maximize cooling efficiency, and they can get heavy.
Load Up the Ice
Before you load your cooler with food, put some ice or a few frozen gel-packs into the cooler and put the lid on it for an hour, so the frozen items don't expend any energy in cooling down the container. Ice blocks and gel-packs last the longest, but if you don't have any, put ice cubes in a plastic bag with a tight seal, add some water and freeze the bag to make an ice block. Have enough ice in any form to fill up any empty spaces in the cooler after the food is packed.
Pack It Tight
Packages containing raw meat belong in the bottom of a cooler where they can't drip onto other foods, potentially contaminating them. If you're only bringing one cooler, put the items you'll use the most often on the top. If you have more than one cooler, pack one with items accessed often such as condiments and drinks. Pack food in storage containers or sealable plastic bags so it stays dry as the ice melts. Put some ice at the bottom of the cooler, and around the food as you pack, but save the bulk of it for the top of the cooler to maintain a consistent temperature because cold air sinks. Pack the cooler completely full to keep food cold longer.
Wrap It Up and Stow It
To insulate the cooler further and keep it cold even longer, wrap it up in some thick towels or a blanket. Keep it somewhere in the main compartment of your car avoid putting it in the trunk where it can heat up faster away from air conditioning. If you're camping, keep the cooler in the shade covered with a tarp when not in use, and access it as infrequently as possible.