Try to remember everything you can about the movie you wish to identify. Think of when you saw it, which might help you place it later. Try to place any of its actors, the director, what studio released it and who you saw it with. If you remember the movie well but just can't think of the specifics, write down any plot details or scenes that occur to you. Even the genre of the film may be enough.
Visit the Internet Movie Database (IMBD) and type in the actor's name if possible. (See References) If you can just place one director or actor, your job will be made much easier. Click on the provided link to that artist's experience and scan through all the films that person is associated with; they'll be presented in reverse chronological order. Even if you can't find the name on this list, there's a good chance that by clicking around a bit on IMDB you'll stumble onto it or at least be reminded of someone associated with the film.
Try to place what studio released the movie. If it's an older movie, such as one from Hollywood's Golden Era, you may be able to associate the film's style with the studio that produced it. If so, visit the studio's site, such as MGM.com, and search through their film archives for the movie title.
Write down as many specifics about the film as possible. If you can remember any key details from the film, try entering those into a search engine such as Google. For example, if you were trying to think of the title of "It's A Wonderful Life," you might type into Google, "Christmas time travel movie." If you dig around in the search results, there's a good chance you'll find what you're looking for.
Consider calling a friend or acquaintance you attended the movie with as a last resort. Text-message sources such as Cha-Cha and KGB allow you to ask any question, and they'll try to find the answer for a price. (See Resources 2)