Ditto this. I keep several sizes of bags in my trunk bag for this purpose (1 each of larger sizes for other items I may have in the trunk bag). Snack sized is best for smartphones because then you don't have to fold the bag back behind the phone before putting it in your jersey pocket. And like others have said, touch screen capability is retained.

There is absolutely no need for those "dry bag" things, or other similarly overpriced plastic bag options.