Camelbak actually makes a cleaning kit, that includes two brushes to clean out the reserve and hose. It also comes with a couple Camelbak cleaning tabs along with a reservoir hanger. The reservoir hanger is key, because after every ride you should wash out and hang dry your reservoir using the hanger.
Here are the cleaning tab directions:
1) Use CamelBak's Reservoir & Tube Brushes, along with soap and water, to clean interior of components
2) Rinse components
3) Refill Reservoir with 1 litre of water
4) Add 1 Cleaning Tab and allow to dissolve
5) Shake the reservoir, then let stand for 5 minutes
6) Empty Reservoir and rinse with fresh water
7) Either refill to drink or use Reservoir Dyer to store Reservoir
Other than tasting gross water, you'll also know it's time to clean your CamelBak when the interior of the bladder starts feeling slimy. From experience, I can tell you this cleaning kit / tabs work well and will get your CameBak water tasting right.
Keep in mind, after the first few times you use the reservoir after cleaning it, it may still have somewhat of a cleaning tab soapy taste... so be sure to wash the reservoir out with fresh water after every use, and dry it out with the reservoir hanger provided in the cleaning kit. Drying out the reservoir by using the hanger after every use helps tremedously to help keep your CamelBak from becoming gross tasting.