get as big a cooler as possible and it should be a light color so it doesn't absorb heat as much
ideally use two coolers
this allows separating your food and drinks so you don't keep opening your main cooler and spoiling your food
drinks don't need to be as cold as they don't spoil so you can get away with a less effective smaller cooler
it also means when you are getting too much air space in your main cooler, you can then transfer your contents of the small cooler back into the main cooler to make it more efficient at staying cold
pre-cool it the night before you lave by adding ice and water which you will then discard before packing it for your trip
prepare your foods before packing
place foods into resealable plastic bags to reduce the risk they will become sodden and also allow for better space utilisation by removing the packaging
any foods not going to be eaten on the first day can be placed into a sealed bag, flattened, and then frozen which will reduce spoilage rate and move the food from the food calculation and into the ice calculation
ensure any foods that will be going into the cooler has been refrigerated FIRST - don't make your ice have to work too hard!
use a ratio of 2:1 of ice to food
use block ice when possible as it will melt slower
if you have to buy crushed ice bags, these are generally not as cold as they could be so consider putting them in your freezer the night before you pack - if you have a freezer that big!
BUT you will ALSO need crushed ice to fill in the air gaps around your drink and foods
use frozen ice packs to layer on top of your food
ice turns to water - don't let you food get sodden, especially chocolate!
pack your cooler well by layering it
put your block of ice and frozen foods at the bottom
then add a layer of your refrigerated foods
then add a layer of crushed ice - this layer should be as thick as the layer of food
then add further alternating layers of food then ice
finally add your layer of frozen ice packs
ensure the cooler is not in a hot area and not in the sun
avoid opening and closing it too often, and don't leave it open for long