It's of course best to cook and eat your lobster immediately, but you can store them overnight in most cases. Unwrap them, they need to breath, and keep in an open container in the coldest part of your refrigerator. Pack with moist (not wet) newspaper. Do not store in contact with ice or tap water and don't let them freeze. If you lobsters die overnight they may still be fine. Dispose of any with strong odors.
Double bag the cooked and chilled lobsters in freezer or vacuum bags. Place in freezer, the colder the better. You can freeze them whole or break off tails and claws. Separating the bodies will extend the storage life. Bodies make great stock and make a delicious base for many dishes.
Frozen lobster will keep 3-6 months depending on freezer temperature. Thaw overnight in the refrigerator to use. To cook, steam for 8-10 minutes.