Bleach works, but you have to rinse it out a few times, and use some baking soda to kill the taste.
Hydrogen peroxide also works, and doesn't leave that nasty chlorine taste. I usually pour in some peroxide, a bit os dish soap and slush it around, then rinse thouroughly.
If/when you ever replace the bladders, get the newer Omega ones with the big mouths. They are about 4 or 5 inches across and if you have small hands you can reach inside to clean too.
For drying the Camelbak brand cleaner works great. It's a plastic ladder like device that you fold, insert and it opens inside the bladder so it can dry all the way. A piece of stainless wire can also work, just bend it into a zigzag shap, insert into the bladder and rotate it so that the plastic doesn't lay against itself. A company (Hydra I think) was selling something like that once, might still be. Doesn't work as good as the Camelbak one though..