Artist Travis Pitts has created a little visual primer for design promo posters for modern Hollywood movies.
Be sure to check out the fine print, which provides helpful suggestions like, "modern children's movies require blue sky, clouds, and the same font treatment as wacky comedy. All that is needed of the character is a peeking tease, and some element to imply that the character is coming "to the big city" or into the real world, and, commonly, in 3-D."
Be sure to view it in large format to read all the hilarious (helpful?) fine print.