You know, the one thing celebrities don't like is being out and about trying to just live their daily lives and not being left in peace. If they are at an "event" you might be able to approach them to say HI and ask for an autograph, but otherwise it is more courteous to just acknowledge in your mind, "Hey, I just saw SoAndSo," and leave them alone.
Los Angeles is a big city and celebrities won't want to draw attention to themselves on a normal day. Your best bet would be going to nice restaurants that have a good reputation because celebrities will want to go to nice places to eat. Whenever my friend in Los Angeles runs into a celebrity, it involves getting food or something to drink.