It's very easy to get to South Beach from airport. You can either take a cab and pay $24 flat fee or take a shuttle service and it costs u about $15.- I suggest to take a cab and pay the difference because the shuttle service can sometimes be a pain on the neck.
If u will definitely use a shuttle service, be sure to be on one of the vans of Super Shuttle (dark blue vans...u cannot miss them) which is the best and cheapest service from airport.
We flew into Ft. Lauderdale because it was much cheaper. There are shuttles to and from Miami Beach, and the cheapest we could find was $55 plus tip. This is cheaper than a cab or a limo service. It takes about 45 minutes to get between this aiport and Miami Beach. I don't have a contact for the shuttle itself, but if you call the Marseilles Hotel concierge, they'll hook you up. The phone number is (305) 695-1010, or (786) 287-1121.