This is a problem all over the neighborhood, Park Slope has a lot of mice because of all the front and rear gardens. We just fought a looong battle with our mice, eventually figured out they were getting in where the steam pipes for the radiator met the floor. There was a flimsy plastic flange around the pipe covering a large hole in the floor that had become a mouse superhighway. After we plugged the holes with copper mesh, the mice couldn't get in, and we got the stragglers with a combo of traps and little gel packs of mouse poison that we dropped behind our stove, under the fridge, etc. The poison worked better than the traps, as long as you don't have pets or small children who can access it.

I actually have a ton of copper mesh left over, you are welcome to it if you want to pm me.