contrary to east-coast opinion, midwesterners can indeed benefit from bags such as this one:
http://www.chromebags.com/products/bags/show/30/
i find it quite comfortable, it holds everything i need to carry (books, laptop, clothes, shoes, etc...) and, most importantly, is entirely water-proof. the shape is quite tall and flat so, if your primary interest is sticking a case of beer into it, this probably isn't the solution for you but it still recommend it as a nice, dry, large-volume backpack. i got mine for $110 at the LBS...