Rothko, I have had a few times in my life when the Somm truly earned something in return, like pulling special wines not on the list. (I always inquire what is not on the list at special places)
I always tip in cash with a simple handshake at the end of the evening. In Europe, I always tip everyone in cash at special restaurants.
A recent glorious trip to Bern's, we had the best Somm I have had in the States. All four of us gave him a $100 each for his personal service.