The following is an e-mail I received from bowshooter.com.
I wish I had thought of this one!
By Robin Conrads
To help foods stay frozen in your ice chest, freeze a brine solution in a reusable container and place it into the chest. A frozen brine solution is colder than ice and will keep food frozen longer.
Fill a clean milk jug with water, leaving about 1" of space for expansion. Add 1/4 cup of salt to the water, replace the cap and shake well. Mark the jug as "brine" or "non-drinkable water" so there isn't any confusion in camp. If you have a large cooler, make two jugs.
Freeze the brine solution, shaking it several times during the freezing process to redistribute the salt. Place the frozen jug into the bottom of the cooler and pack frozen meat and other frozen items around it. Add more ice and the other items that don't need to be frozen.
Keep the cooler in the shade and open it as little as possible. It might even be wise to have a second cooler for non-perishables, like canned or bottled beverages, so you aren't getting into the frozen food cooler as much. Remove the meat and let it thaw before the planned meal. If this works properly, the meat will not thaw in this cooler.
There is no need to make this solution every time you use the cooler. Just put the jug back into the freezer when you get home, and it will be ready for the next trip.
Thanks from www.bowshooter.com