Knowing this is old but I will add my process, which includes several of the ideas already posted. Using this process we get a full 7-8 days in moderate temps in our Coleman marine cooler.
- 2 days before departure I bring the cooler into the house from the garage. This is important since we live in Texas and the garage gets pretty warm.
- The night before departure I dump in a couple bags of cube ice to condition the cooler
- Any freezable meats that we will not be using in the first couple of days are frozen
- We don't take a lot of soda, mostly bottled water, so I freeze at least half of the water and the rest is refrigerated until loaded
- For the primary cooling on the trip I save, fill and freeze 4 apple juice bottles (3 qt size)
- Day of the trip I drain any water, put the apple juice bottles in the 4 corners, then load in everything else
- Frozen meat is put into freezer bags to prevent from getting wet, that and the frozen bottled water go to the bottom of the cooler, everything else sits on top of that.
- Once the cooler is loaded I backfill with cubed ice on top if I have extra
While camping we try to minimize the opening of the cooler by always asking if anyone needs anything when opening it for ourselves. I know this sounds trivial but it cuts down the opening in half, at least. I like to use the apple juice bottles instead of milk jugs because the lids are more secure and they just seem to fit in my cooler better. And once the milk jugs start to melt some, jostling of the cooler sometimes pops the lids off. When filling the apple juice bottles I make sure not to top them all the way off (leave an inch or so on top) and before closing the lids I squeeze the bottles to bring the water level all the way to the top (getting out all of the air). This help keep the bottle from deforming as the water expands when freezing, and eliminating the air helps the same since we are usually going from about 500' elevation to 11,000' elevation.
We have also stopped taking beer on camping trips due to the amount of space they take, the weight of the bottles (my wife and I don't care for canned beer), and the trash that results. If we are going to be drinking we take liquor and / or red wine, so we don't use cooler space.
As I said, this process nets us a good 7-8 days, and when we get home the apple juice bottles are still mostly frozen.