I start with clean water, and on a hot day I'll fill the bladder first with ice cubes and then with water.

When I get home I'll rinse the bladder out thoroughly with clean water and then make sure it dries completely. I do that by dumping out all the water, hanging it open upside down, and then cleaning out the water that gathers near the opening. Once that's done I will stuff a clean paper towel up into it to allow air circulation. Part of this process involves taking the mouthpiece off and allowing plenty of clean water through the tube before hanging it up.

Using Camelbak bladders containing only clean water and always put away clean and dry, I've never had a problem with any visible bacteria or musty tasting water.