A viewer sent in the simple idea of pushing finishing nails through a piece of corrugated cardboard to hold them steady while you come down with the hammer. That means that you don't come down with that hammer on your fingers. They you simply pull the cardboard up and finish driving the nail.
Let me add another tip that I have used for years. If you don't want to see the putty covering the finishing nail head, take a round gouge and lift up a "sliver" from your molding, drive in the nail below that sliver, and then glue the sliver back down. A spot of white glue and some masking tape as a clamp should do the trick -- no more nail at all.