Size: It pays to go a little bigger, than too small and aim for a 2:1 ice to contuse ratio as a rough rule.
Prep: Do your best to make sure your cooler box and contents is nice and cool before loading it. cooler boxers can hold internal heat even if the surface feels cold!
Cleanliness: give your cooler box a spray down with disinfectant it give you food the best chance of staying fresh
Ice: the big pieces of ice are helpful as they have less surface area. you can also freeze salt water in bottles and put among the ice. these will melt first but will cool plain ice as it does
Short Trips: trips around 2-4 days Ice packs are good option but are once there melted they are just taking up space
Longer trips: 4 Days and more its a good idea to go with ice as you can keep adding to it and drain the melted ice as you go
Loading: Large bits of ice and the bottom. then some small bits. the less air gaps the better. lastly nice flat freezer packs on to to trap the cool in when the lid is opened!!!