When playing or working with my dog, I use methods learned from watching Cesar Millan. He says one way to keep your dog from chasing things is to "claim" them. In other words, place yourself between your dog and the object of it's desire while having an attitude of "that's mine". For example, "my bird", "my squirrel", etc...
While out for a walk this morning I found myself thinking "my bunny...lies over the ocean"