If you put a live lobster into a pot of boiling water, it can thrash and twitch for up to 2 minutes before it is rendered motionless. The best and most humane way we've found is to first freeze the lobster for 30 minutes, inducing a coma-like state.
Cooking times based on size of lobster:
1lb - 12-15min, 1.5lb - 15-20min, 2-3lb - 20-25 min, 3.5-5lb - 25-30min
To avoid overcooking, only similarly-sized lobsters should be cooked together. If there are various sizes in the pot, base the cooking time on the largest lobster to ensure that it is cooked.