3 way switch and an "ice cube relay". there are times I have run into problems with remods and a existing light circuit with a single pole switch needs to become a 3-way circuit and the existing single pole isn't excessable to change the wiring. I've got around this by using a ice cube relay in an approprate (excessable) box with the "old" switched run powering the coil. wire the C/NC/NO as a 3-way switch with the NC/NO as the travelers. Our local inspector couldn't find any reason to reject this creative idea. Only problem I have seen is it confuses some people on how it works. Also solves problem when the old circuit doen't have a ground and the new lighting needs one. In this case think of the old circuit turning on and off the relay (now acting like a 3-way switch) electrically as you would a normal switch by hand.