Put the lobster in the freezer for about 30 minutes, it will basically be in a sleeping/coma state when you take it out. Then cut into its head lengthwise to destroy its brain before it wakes up.
Some people say that killing the lobster in a less stressful way gives the meat a better texture/flavor. But personally I have not been able to tell any difference.
EDIT: Another solution, though not a clean or popular method for lobster; If you destroy its brain faster than the speed pain travels through nerves, it would be dead before the pain reaches the brain. Could be accomplished by shooting it in the brain with a gun... a common method used to humanely kill many things.
The above link details a series of experiments done by Dave Arnold and Nils Noren on 7 different methods of killing lobsters, including the freezing mentioned by Netduke's answer. The article concludes that (when it comes to flavor at least)
Anesthesia [is] (still the best)
I can't speak to the objective difference in humaneness of killing method between this and something like freezing, but I suspect that isn't a question that's answerable.
I know Mom loves lobster and she swears by Steaming lobster. It was also the preferred method when I worked at a very popular Restaurant Chain.
Mom would start with warm water in the bottom of a pot. Then she put the lobster in on a rack to hold it clear of the water. Then she increased the heat, Pot uncovered, and timed it for something like 10 minutes after water came to a boil (putting the lid on the pot). She claimed the slower rise in heat knocked it out. All I know is the the lobster usually stopped moving when the first bubble formed and it never thrashed violently.
I can't be sure when exactly it died and whether it felt pain, but it seemed way better than just dropping it in boiling water.