The key to successfully storing a live lobster is remembering that it has unique physical features that require some consideration. First, a live lobster is a living, breathing creature, and to ensure that it remains the former, the latter quality must be accommodated. Lobsters breathe through gills, and these must be kept moist in order to work properly. There are two things in particular to avoid in order to keep your lobsters alive until you are ready to cook them: heat and dryness. Keep your lobsters in a cold, damp, and suitably oxygenated environment and you can expect them to survive for approximately 36 to 48 hours after you bring them home.
Select lobsters that seem active and alert. Make sure the lobsters have elastic bands around their claws so they don't injure you or each other while in storage. The color of the lobster itself isn't significant. Lobsters may range from brown to black to blue or green.
Handle your lobsters gently; the less handling, the better. Lobsters may be stressed and weakened by excessive jostling.
Place the lobsters in a large container, such as a Styrofoam cooler, without a lid. If you are packing a large number of lobsters, alternate lobsters head to tail in the container, curling the tail beneath the lobster. Place damp newspaper strips or seaweed between layers of lobsters if you are packing them on top of each other. Careful packing will help to protect the lobsters from injury. Cover the lobsters with seaweed, damp newspaper torn into strips, or loosely draped damp towels. If possible, use seawater to dampen the newspaper or towels.
Check to make sure the newspapers or towels are still damp when you get home. If there is room, place the entire container holding the lobsters in the refrigerator. If not, clear out one of the vegetable- or fruit-keeper drawers, line it with a damp towel or newspapers, and put the lobsters in that. Re-cover the lobsters themselves with whatever damp covering you originally had over them. If using a container, do not close it with a lid, as the lobsters will need air to breathe and will suffocate in an airtight environment.
Do not immerse live lobsters in fresh water or salt water and do not place them on ice. Keeping them in the refrigerator will provide a cold enough environment, and surrounding them with damp material will keep their gills sufficiently moist. Ideally, you should plan on cooking and eating your lobsters as soon as you bring them home, but they should survive nicely for a day or two if you provide hospitable conditions.