I'm not sure how the case fits into the discussion, since the lens for the camera(s) is exposed, case or no case.
But your question is a good one: what kind of lens cleaning device works best. I've been using a VERY CLEAN Q-tip, and it works fairly well. Also, a very clean, dry flannel cloth works well on the screen, and would work well on the lens.
I don't have a problem with mine. I use a case but as EWyatt explained the lens is always exposed anyway. I've had my phone for a few months now and never had the need to clean the camera lens...
A tip from a photographer: Use a proper lens cleaning tissue, just breathe out on the lens (not when you are eating ice-cream) and use the lens cleaning tissue on the lens in rotation. Hope this helps