After you sign up for courses, it helps to check every week or so to see what has opened and filled. People switch their classes around right up until the last moment, so you might find a spot in a class that you want.
The classes that I’ve typically seen fill up at World are the TRX classes. I’ve even gotten into those when the class is full and the wait list is full. They let the stand-by line in to sit around the edge of the room and observe. Since I’m a visual learner anyway, that was just as good as doing the workout because I like to take notes.