Just throw them on ice as soon as you catch them, while they're alive. Keep them on ice until you're ready to clean them, and they'll be fine. I've seen some folks on long fishing trips put their fish on dry ice, but I wouldn't recommend that. That will freeze them hard as a rock, so hard they'll get freezer burn! Plus that stuff can be dangerous. And if they're of any size (bigger than the stockers they put in the local ponds) let me suggest you do a butterfly fillet on them, and stuff them. Yum!