Several technics will prolong the cooling time:
Pre-cool. It works for the food and beverages, as well as for the cooler itself. Although it’s hardly possible to place a whole thing in the fridge, it’s better to leave it in a cold place for several days before using it.
Use a block of dry ice. It’s a possibility only for a rotomolded model. Dry ice lasts longer and keeps your contents cool for an extended period of time.
Cool drinks before using the cooler. It’s preferable to chill all the contents overnight before putting them in the cooler.
Layer the items. It’s better to use layers, switching ice, and food for a better chilling effect. The best ratio is 2:1 for ice and contents accordingly.
Don’t leave it in the sun. As the chill box is getting warm while sitting in the sun, the cooling capabilities will expire faster than they should.