Holy crap. I've been here in LA a little over 3 years without finding an authentic Philly cheesesteak, but that all changed the other day. In Burbank, CA there's a place called "Philly's Best" on Olive Avenue and their Philly steak sandwiches are
dead on. They even have the bread shipped to them from back East to make sure it's 100% authentic and they have a few framed t-shirts from Pat's, Geno's and Jim's steaks in Philadelphia as if to pay homage to their Philly cheesesteak mentors.
So if you're in LA and in need of a good cheesesteak, take it from a former Philadelphia resident and go check that place out pronto! (I recommend getting one "with wiz" btw)
http://www.eatphillysbest.com/