When I was living aboard (Toronto winters). Some people would hang a 60 watt bulb and fixture next to each through hull to keep then nice and warm over the winter! I did this for a couple of winters and then realized that this technique wasn't necessary with the bubblers in our marina! The system they used had massive compressors and the entire slip would stay ice free.
Even if the slip stays Ice Free due to any kind of bubbling system, (air, agitation, ect) the thru hulls will still freeze if the temp in the bilge gets below freezing.
The light bulb idea is OK, as long as the bulb doesn't burn out, but it's not foolproof.
The most effective way I've found is to pour vegetable oil into these (drains, or inlets, or whatever), and close off the valve while you're doing this. The oil will displace any water in the thru hull fitting, and since oil is lighter than water, it won't seep out the bottom, below the valve. It will actually coagulate within the thru hull fitting as the temp gets colder, and provide a nice sort of soft plug while keeping water out of there. Also, if vegetable oil is used, you won't be accused of polluting the marina like a normal oil or gas spill.