I got the train (www.amtrak.com) from LA to SD before. Air Con, very comfortable and apparently a breathtaking view in the light (I travelled at night time).
depends how much time and money u got...........the cheapest way would be to get a greyhound bus-cheap and cheerful with colorful clientelle.......when i did it i rented a car coz the route takes u along the coast and tis nice with places to stop along the way such as monterrey and santa barbara........
Spend a few days driving from San Fran to LA along route 1. Amazing scenery. Did it Last year year in 2 days but probably need a few more days to do it justice.
Definitely the best way is by hiring a car although the one way rental cost may be expensive.
I drove from San Francisco to LA last year over a couple of days following thye Pacific Coast Highway (Route 1).
There are sections where you literally hug the coast going in twists and turns looking over a sheer drop to the sea; some brilliant photograph oportunities too.