If you're wanting to avoid the enormous premium of buying prepared crab meat for a salad, you should know that a live crab will lose almost one third of its weight when you steam it, and you'll get about one third of that as crab meat when you remove the meat from the shell, so a 1.5 lb. crab will yield no more than 6 oz. or so of crab meat.
Boiling:
Bring a large pot of water to a boil and add salt. Place your live crab or lobster in the pot and put the lid on. When the water returns to the boil, cook for 10 minutes for the first pound and 2 minutes more for each additional quarter pound.
Steaming:
Place your crustacean on a rack over boiling water and cover tightly. Steam for 15–20 minutes, depending on the size of the beast; if it's under 2 lbs. 15 minutes is enough, over that and you want to cook it for 20–25 minutes. If it's over 3 lbs. it's probably too old and tough to be worth eating.
Serve with a baguette and hot lemon-garlic butter.