Also works great when going to multi-day music festivals with no power on the campground. We always buy some 20kg or so of dry ice blocks, a big styrofoam box full of it, and move a block over to the cooler containing the beer every few hours. Works great for up to 5 or 6 days.

Also, putting little bits of the ice blocks directly into drinks makes for a cool fog effect in addition to keeping your gin tonic at refreshing temperature.