I discovered this neat trick an hour before setting off on a climbing holiday after finding that my CamelBak hose was disgustingly dirty.
Being loathe to spend £20 for a brush on a wire to clean the hose and without time to get one I looked around for something that could clean the hose.
Paracord was the answer! A loose knot in the end of a length of paracord, dropped through the hydration bladder's hose can be drawn through slowly and bring all the gunk with it.
Here's how I've done it ever since.