The most common way of killing a lobster is to put it into boiling water, however we can read on Wikipedia page that it is one of the methods which cause pain and distress.
The killing methods most likely to cause pain and distress are:
Placing crustaceans directly into boiling water
Placing crustaceans in slowly heated water to the boiling point
Placing marine crustaceans in fresh water
Secondly, we can read here about the following study:
Dan Ward, then a grad student at the University of New Hampshire, once hooked an electrode up to a Lobster heart for me and boiled it. The heart beat for 1 minute 53 seconds.
So the question is, what is the most humane way to kill a lobster?