Colder is always better, this year I switched to one of these and so far so good, I do try and keep a couple frozen water bottles in there as well to help keep it cool.

http://www.cabelas.com/product/Engel...Q&gclsrc=aw.ds

That is just an example, some people use a cooler that they modify with an aeration kit and Frabil has an insulated bucket type as well, all work in insulating the water, throw a frozen bottle in there and you should be good. A non insulated bait bucket in my experience does not keep them alive too long if it is hot out.