Few tips that I have read about over the years......some others have probably already touched on some of these topics......
1. Always pre-chill the cooler. Best option is to put the cooler in a walk in freezer overnight, but a "sacrificial bag of ice" will do the trick.
2. Use block ice, when feasible and then fill in the "holes" with cubed ice
3. Limit opening the lid of the cooler as much as possible.
4. Keep your cooler as full as possible, b/c it takes more energy to cool empty space.......or when you open the lid, hot air gets in the gaps and then has to be recooled by your ice........
5. You can lay a wet towel over your cooler and evaporation will help keep the overall temperature of your cooler lower.
6. As Glenn said, all ice is not created equal. If you buy your ice and it looks "wet", then it will not be as cold or last as long as hard, "dry" looking ice.
7. Freeze any items that you can before you put them in the cooler.
IMO, you got to have 2 coolers. I would rather not have a fishy beer.