1. A white background is essential. It symbolizes the vast abyss where a plot should be. Add a slight gradient if you're feeling dangerous, but be warned, cityscapes (Just Like Heaven, The Object of My Affection) and blurry whirlwinds of emotion (Forces of Nature, Serendipity) are really advanced stuff and not to be taken lightly. Like witchcraft.
2. Add protagonist(s) looking coy and flirty or shocked at love! Hands on the hips for extra sass.
Add couple and position them in a way that's a metaphor for their relationship. Can't go wrong with a classic "back-to-back" or any other pose commonly found in prom pictures.
3. You're allowed one prop. It can be flowers, bags (purses and luggage included), or an animal.
4. Title that bitch up! Remember: the human heart is red and pumps 74 gallons of love per day; therefore, your text must be red or pink. Science.
5. The catchphrase is the most important part. Without it, how would the audience know that, "The only risk in taking an adventure is not taking it at all," and other things that don't make sense?
Ta-da! A perfect movie poster! One of a kind...of.