Found this online a few years ago when faced with the same issue...
"Tips that save money! Whatever brand or size of ice chests, icebox, esky or cooler you own, these tips will help you keep your ice longer so you can get the best out of your camping trip or backyard barbecue. 1. Prepare your icebox. Spread a layer of crushed ice around your ice box the day or night before you use it. A bag of crushed ice costs around $5, but don’t worry it won’t be wasted. By cooling down your icebox, and the air inside, in advance you’re doing half the job of the main ice you’ll put in later. 2. Use block ice. Crushed ice is full of space and air which means there isn’t much actual solid ice, so it melts quickly leaving you with an icebox of cold water. Block ice is a solid mass of ice. It will keep your icebox as cold as crushed ice but won’t melt as quickly. When block ice is unavailable, make your own using ice cream containers or juice bottles remembering to only part-fill them to allow for expansion. 3. Add some salt to your ice. Adding salt to water before freezing lowers the freezing temperature of the water, meaning that your ice will actually be colder than frozen fresh water. Using sea water will work even better than adding your own salt to the water. 4. Cool your drinks and food first. No matter what you’re putting into your icebox, cool it down first if possible. By taking your food or drinks from the fridge straight into your icebox, you’re saving your ice from having to cool the beers down in the first place, making the ice last longer. If you don’t have room in the fridge, put your beers in the crushed ice you’ve already put in the icebox. 5. Adding the Beers! If you’re putting drinks in your icebox, leave the crushed ice in (from Tip #1 above) even if it’s already half melted. The cold water will help to slow your block ice from melting. 6. Keep it out of the sun! The sun is your worst enemy in preserving your ice and the reasons should be obvious. Keep your icebox in the shade as much as possible and ensure there is good air flow around the box. Sitting the icebox inside your tent or car is like putting it in an oven as temperatures can often climb 10 – 20 degrees higher than outside. You could even cover your icebox with a blanket or towel to shade it from the sun, and if at the beach a wet towel will work even better. 7. Avoid opening your icebox too much. This is obvious, but every time you open your icebox not only do you let the heat in but you let the cold out. Avoid opening your icebox too much. 8. Fill your icebox as much as you can. An icebox packed to the brim will preserve its ice longer than a part-filled icebox of air. The more food or drinks you have in your icebox, there is less air which needs to be cooled down and kept cold.
