Believe it or not Sunset Blvd. is a great place to run into celebrities. A lot of agents have office's on Sunset Blvd. and celebrities have lunch in that area. The Hamburger Hamlet at the west end of Sunset Blvd. is one place, because it has large booths and it is dark. Sometimes you can even over hear producers and directors talking about a movie they are working on. A rule of thumb is to not interrupt a celebrity while they are eating.
There are events that will guarantee that you will see stars, like a ceremony on the walk of fame or award shows. See the link below for a calendar of events. For example, the day of the Oscars, people come from all over and converge on the area of Hollywood Blvd. and Vine. They either circle the area or park their cars and walk. The closer you get to the theater the more likely you will see a celebrity. It is very, very busy and crowded and hard to see sometimes, so you may want to choose a smaller event. You may not be able to get an autograph, but you will be able to get a picture and see them.
Rodeo Drive in Beverly Hills is where a the more famous stars shop. If you see a security guard in front of a shop and no one being let it, then there is a star inside. You can try looking into the window or wait till they come out to see them. As you are looking, remember you may not be able to tell who they are as some stars will dress down and put on a baseball cap and glasses to hide their face. So, if you think it's so and so, it probably is.