I have this hip pack for long days, winter riding, and trips on the tandem:

http://www.inov-8.com/new/global/Pro...ro-4.html?L=26

It holds a 2 Liter bladder, wind jacket, insulating layer, warm gloves, tools, snacks, wallets, maps, phone... in case you'd ever need to take your entire household with you...

No sweaty back, chafing, or unnecessary shoulder tension, and I like that I can access the front pockets while riding. If you seal the seams it's also waterproof.