I do low-voltage.
Most of the stuff I see here about drilling a small piece of wire through floors and walls in order to find it on the next floor I accomplish using a toner/tracer.
Clamp the toner leads to a short piece of coat hanger or simular, leave that on the floor next to the wall where you want to drill.
On the ceiling below use the tracer to find it, measure over 6cm and you'll hit the wall cavity.
For drilling down from the attic, I mostly work on new houses.. so all the top plates for the walls are clearly visible between the drywall in the attic. Before I go up I just remember where all the electrical outlets and switches are in relation to where I want to place a wallplate. Then in the attic I look for the top plate, find the wire drops for the electrical, measure over and drill and drop a toned line down the hole. Back in the room I trace the wall for the tone, cut box hole and pull the cable through.
