I never clean mine. Never had mildew. I wind up replacing them every 12-18 months l, not because they get mildew, but because they wear out and leak after prolonged, heavy use.
I drain mine after every day on a hunt or every hike. I Only put water in. I have one of those camelback drying hangers. $8 for a piece of plastic is silly but it sure has seen its use. Open up the valve on your hose, blow the water out hard, and let it dry before close and reattach. I do wash the mouthpiece with soap snd water.