I have a seat wedge for a spare tube, co2 cartridge, multitool and levers, and $20. In my jersey pockets I carry my cell phone and snacks (and now my digital camera, as I only just bought it). For rides of 50 miles or longer, I have a handlebar baguette bag that holds a 12 oz bottle of Gatorade, additional food, a handkerchief, sunblock, extra chami cream and another $20. I have two bottle cages on my bike, so I'm carrying 48oz regularly and 60oz on long rides. My 50+ mile ride routes all have at least one store along the way, so I can stop for extra liquids.

I've tried riding with a camelbak and a messenger bag. They both suck! I don't recommend it. I now have a lumbar camelbak for hiking, which would probably be better than the backpack camelbak, but it would interfere with accessing my jersey pockets.