You'll always need somewhere for the niggly bits - the tool kit, the spare gloves, the pump, maybe even the tent poles. Enter: the frame bag.
Topeak's offering is the Midloader, here with 3 litre capacity. Constructed of lightweight, highly water resistant and durable nylon, it'll keep your stuff tidy and dry in pretty much all weather. Two water resistant zuppered openings provide easy access from either side.