Here's a tip for those of you that freeze your own ice in pop bottles or in milk jugs... if you put salt water in your bottles and then freeze it, it will keep your stuff colder for longer.
If you want to get technical, you could make a eutectic solution, basically it's 1lb of pickling salt to 50oz of water. This stuff will not freeze unless your freezer is colder than -6F, but if your freezer is cold, it works great. It takes a while to freeze it in the freezer, but it also lasts a LONG time in the cooler.

-N.