Boiling is more common, but I prefer steaming crabs - they emerge from the pot less water-logged and more filled with their own crabby juices.
To Boil: Bring a large pot of water to a boil. Add enough salt so it tastes as salty as the ocean. Add any spices or herbs you like (Old Bay has plenty of adherents, for example). Add the crabs and simmer until they are cooked through, 10 to 15 minutes depending on the size of the crabs.
To Steam: Bring about an inch of salted water to boil in the bottom of a large pot. Set a steamer basket in the pot and the crabs in the basket or, if you have a lot of crabs, simply pile them in. Cover the pot and steam until the crabs are cooked through, 10 minutes for small blue crabs and 15 for larger Dungeness and up to 20 minutes for even bigger crabs.
See more about How to Cook Crabs here.