How can I prevent water from remaining in a flexible shower hose?
i would have to disagree with daniel above.  standing water leads to mineral deposition and galvanic corrosion much faster, and in places it may not normally happen.  pathogenically, you could also be hosting lots of little bacteria and fungii in that standing water, particularly on a well water connection.  hand wands sometimes go months between uses, so i think you are probably right in trying to avoid this.  

in these installations, it depends on a few things:

1)  loops in the wand hose will hold the most water.

2)  some wands have a backflow valve in them,  this will prevent any break in the column and could leave water in a hose no matter what you do.

3)  some tub diverters don't open properly on stop of water.   they pop open when the pressure comes on again, but dont necessarily release when the water flow stops.  that could hold water above them.

4)  some thermostatic mixing valves (like the moen 3285 and 3210) are notorious for not letting water flow back through them freely or predictably.  this is just the valve design and cant be worked around.