LA has a chinatown downtown, but don't waste your time there. The San Gabriel Valley (Alhambra, San Gabriel, Monterey Park) has some of the best Chinese food in N. America (or go to eastern Hollywood Blvd. in Thai Town for great Thai, or Koreatown for great Korean food). When you get to town, go to a bookstore and pick up Counter Intelligence by Jonathan Gold, a collection of his non-touristy/local/ethnic recommendations from his reviews when he was LA Times restaurant critic. Here's a link to his current list at the LA Weekly, searchable by type of food, price, locations:

http://www.laweekly.com/food/totallist.php