By car: take Century Boulevard exit (and several more northern exits) on Interstate 405, or the Sepulveda Boulevard exit on Interstate 105.
By bus: many routes (local, rapid and express) of the LACMTA, Line 8 of Torrance Transit, Line 109 of Beach Cities Transit, and the regular as well as the rapid buses of both the Santa Monica Big Blue Bus system's Line 3 and the Culver CityBus's Line 6 all make stops at the LAX Transit Center in Parking Lot C.
Taxis and private shuttles: are obviously the most expensive option.
If you want to rent a car, there are around 10 different companies with very frequent shuttle buses picking up on the lower level around all terminals and going to large off-site lots. If you want to compare prices, you will need to do so using the telephones in the arrivals area or in advance of arriving. Don't expect any details from the shuttle drivers, or negotiable prices once you arrive at their lot. Signing up to one the car rental club schemes can get the shuttle bus to drop you at your car, thus saving substantial time.