CamelBak have a great answer in the help section of their website.
Summarising their recommendations: Keep it clean and dry when not in use.
If mould develops: Use hot water and two tablespoons of baking soda or bleach Leave for about 30 minutes Wash with hot water and mild soap Air dry
outdoors.stackexchange.com/users/6/Graham
I clean it out then leave it to dry for a day or two.
But you can never seem to get all of the water out of the hose, so I keep mine in the freezer.
At -18 the mold can't grow.
outdoors.stackexchange.com/users/152/Simon Hodgson
Leave the filler cap open to allow any water residue inside to evaporate and so the humidity level of the air inside the bladder matches that of the air in the environment around it
outdoors.stackexchange.com/users/148/mcourtney
This is why I like the pouches that have a zip-lock bottom as well as a cap on the top - it makes it a lot easier to clean the inside and leave it open to let it fully dry.
My friend also swore by leaving the pouch in the freezer when not in use - this would obviously slow down mould growth considerably in between uses.
outdoors.stackexchange.com/users/78/HorusKol
I used a blow dryer on low setting to dry the bag and tub, and a paper towel pouch (twist tie close) with rice - I figure if it gets moisture out of a cell phone and salt - it ought to work for a Camelback.
I also save the little silica gel packets that come in vitamin bottles and new shoe boxes and may try those.
outdoors.stackexchange.com/users/2692/DR.Hiker
I would suggest: leaving the cap open, letting it air dry, then sealing it up and keeping it out of direct sunlight in a dark area (perhaps inside your pack).
When you are ready to use it again,
rinse it
a few times - run some water through the hose as well.
outdoors.stackexchange.com/users/5/studiohack
I get food-grade 3% hydrogen peroxide (NOT 35%) from my health-food/ vitamin store and put a cap-ful in any water container then fill it to the brim.
It works really well to kill microbes of all kinds by oxygenating the water, and you can drink it in that low of a concentration (it is used in commercial food preparation, and some people believe that drinking oxygenated water is actually good for you as well).
outdoors.stackexchange.com/users/2696/blue9
I just get it mostly dry and then pop it in the freezer until the next use.
This keeps anything from growing and takes 30 seconds and no cost.
outdoors.stackexchange.com/users/9626/Danny Zincke
I use a reservoir dryer to hang my reservoir open and upside-down, and... Only ever fill with water Empty and dry every night, even if using it the next day After about half an hour of drying pinch the bite valve and shake to remove any extra water, and also shake the reservoir with the opening facing down to remove any water that has collected Pack it all away after about a day, or once things look nice and dry I've been following this approach with the reservoir for the CamelBak
I bought in 2005 is still going strong, with no mould or other nasties inside!
outdoors.stackexchange.com/users/14700/Richard Ev
When using a CamelBak (or other hydration bladder system), it is easy for mold to grow if the bladder is left sitting with water in it. What is the best way to avoid mold (or other nasties) growing inside in between trips?
How do you prevent mold in CamelBak hydration bladders?
outdoors.stackexchange.com
<cleaning><hydration-bladders><mold>
outdoors.stackexchange.com/questions/630
outdoors.stackexchange.com/users/36/jrdioko
Clean it, leave the cap open, and leave it to dry. A reservoir dryer can be used to hang the reservoir open and upside down. Some people dry the bag using a hair dryer and bags of rice, or even store the bladder in the freezer to prevent mold growth. You can air dry, use reservoir dryers, freezers, blow dryers.
Leave the cap open and let it air dry. Reservoir dryers, freezers and blow dryers can be useful.