Scholastic‘s contest is open to children in grades 4-6 from the United States. To enter, you need to print this entry form (.pdf), fill it out, and mail it in by Oct 29.
More information: The Official Site, The Official Rules.
Focus on the Family‘s contest is open to those 18 years and older, and also in the United States. To enter, you must write in 200 words or less who your favorite character is from The Chronicles of Narnia, and why you’d like to attend the London premiere.
More information: The Official Site, The Official Rules.