I voted 12 ga (slugs) but wouldn't mind a good .375 H&H
Seriously, its hard to prevent something like this I would think. When I am camping there are MANY times (read: most of the time) when I don't have a long gun in my hands, ready to fire... which is why if you are truely serious about avoiding being breakfast/lunch/dinner you should bring a dog. At the very least he serves as an alert.