Take it apart to get at the top and bottom pin. Clean gunk off with alcohol. Use some metal polish to shine them up. Since you don't see it con't worry about how it looks.
Also, don't forget the
threads, inside and outside, where the switch and top screw into the tube. If the type is in pieces that screw together take them apart occasionally and clean those threads, inside and outside, occasionally, they will not get as dirty since they are closed all the time.