Look at the image on the movie poster and try to get a sense of the mood it conveys. Grim films tend to use disturbing images intended to evoke fear or anxiety. Epic movies convey a sense of grandeur, such as broad landscapes or a ship in outer space. Comedies may present an absurd situation, while romances may depict two lovers gazing into each other's eyes. Pay attention to the color scheme. Harsh or intense colors tend to represent a moody or intense film, while comedies and light action films will have brighter, more upbeat colors.
Note the presentation of the film stars. If their names appear above the title of the film, or their faces are prominently featured in the artwork, the studio wants you to know who's in it. (You'll rarely see a Brad Pitt or Julia Roberts movie that doesn't feature them on the poster.) Film posters with no names above the title, or those with more abstract imagery such as cars or explosions, are selling something other than the cast.
Consider the tag line or catch phrase. A comedy pitch line will be witty or clever. Action films stress excitement and adrenaline. Films based on novels may tout the original book, while films by successful directors or producers may emphasize their earlier works.
Look at the film's release date on the poster. Many movie campaigns slowly roll out their coverage, and the period of time before the opening may clue you in to how the studio wants you to feel. "Teaser" posters released early in a campaign will often be mysterious or intriguing, intended to entice your curiosity without revealing too much. Posters later in the campaign will be more explicit, possibly revealing details about the plot.
Examine quotes on the poster. Movie posters love to cite various different critics extolling the virtues of their film. The advent of the Internet, however, has rapidly increased the number of critics out there, and some are more reputable than others. Praise from Roger Ebert, the New York Times, or the Wall Street Journal carries an air of respectability. Films that have fewer good reviews to choose from will often look farther down the ladder for good quotes, citing sources you may be less familiar with. The worst posters use "quote whores"--so-called critics (some from legitimate outlets) who cheerfully deliver indiscriminate praise for even the most dreadful movies. Pete Hammond, Ben Lyons and Shawn Edwards are well-known quote whores, and any film prominently featuring their praise should be taken with a grain of salt.