How does one clean a Camelback?

There are plenty of sites out there that'll give you instructions, and there are also over-priced cleaning kits that you can get at sporting goods stores. Most of the home remedies include using a dilute bleach solution. You'll have that Camelbak tip-top soon enough.

Definitely use DILUTE bleach, like no stronger than 1:10 bleach to water, to get rid of the mildew. Fill it up, run some thru the hose, and close it off and let it soak until you can't see any mold/mildew. (At least an hour.) Then, so your water doesn't wind up tasting like bleach, make up a solution of baking soda and water. Use 4-5 tablespoons for a gallon of water, and pour it in, run some thru the hose, then close it off and let it soak for a few hours. Then rinse it out a few times, and follow the advice of the previous poster to leave it open to dry. Let it dry for a few days so it is TOTALLY dry before storing it again. Hope this helps ;-)

Also, good ways to clean your camelbak before it gets that bad include using vinegar, or dental cleaning tabs (make sure not to get the minty kind unless you want your water to taste minty)

As others have said, bleach. It'll break down ANYTHING organic. Someone mentioned 1:10 bleach/H2O, personally I'd go stronger. I've used 1:2 for a short (5 min) wash w/o any problems. Be careful, at that concentration it'll bleach your clothes too. Then do an extensive rinse, soak it in fresh water for a few hours, hang to dry. That's just what has worked for me, I'm not promising that your CamelBak won't leak/turn to dust/assplode if you use a stronger solution, that plastic is pretty tough though.

A lemon juice rinse helps with the taste of the water.