Study popular movies in your film's genre. Make note of what you like about their posters, what attracts you to them and how they catch your eye. Incorporate strong ideas into your design and be ready to hire a professional for artistic follow through.
Grab a viewer's interest with images of your actors or a primary plot device. Raise curiosity with concepts that relay your movie's basic premise without revealing too much of the actual plot.
Balance your concept to attract both fans of your genre and work with casual movie-goers who may not be aware of who you are or have a particular interest in your style of film. More serious fans need less incentive to buy tickets, while others need a solid push toward a decision to purchase. Keep the focus on making your poster interesting and attractive without needing to cater more to the hardcore base at the expense of other consumers.
Blend the style of your poster with the style of your movie. The poster should relay the feel and ambiance of the film, and remain consistent in its message, especially when it is used in your other marketing efforts. Give the image the flavor of your genre, and let that idea carry through the film and onward to further exposure with home media and promotional materials.