When I was visiting Los Angeles a few months ago en route to Australia, I used Super Shuttle. It was interesting because all of the men who flag down the passengers, and organize the baggage and entering the shuttle vans were black. And all of the drivers in the vans were Hispanic. I asked the driver about this and he said that is the way it is all over the system.
But taking a shuttle to the airport is easy and can be cheaper than parking. For a complete guide to shuttles all over the US, visit this website, Rideflyreservations.com