Depending on the model, take the battery out when not in use. Unfortunately for me, my awesome HTC One is sealed up tight. D:
I also have the HTC. I considered pulling the battery, but would prefer a quick fix, like a shielded enclosure.
Keep in mind that a number of phones have been produced which “work” with the user battery removed and no obvious source of power. Of course, there is a power source; it’s just not obvious (it is a battery “camouflaged” as another component). Whether these special phones have ever been sold as consumer models is unclear.