Read through your story. Copy down words or short phrases that are striking, suggestive or central to the plot. Even a snippet of dialogue or a symbolic image can be useful. Some titles can reveal the setting or main character, especially if the names are unusual. For example, "New York" isn't a very interesting title, but "Chippewa Falls" is more evocative.
Compose a list of the best possible titles. Include ones that are unusual or not immediately related to the story. Edgar Allen Poe's "The Tell-Tale Heart" and William Faulkner's "A Rose for Emily" feature titles with objects whose significance is revealed as the story develops. Titles should hint at the mystery but not reveal everything all at once.
Examine the list of titles, briefly putting the story out of your mind. Select the most promising, the titles that by themselves intrigue or suggest a possible world.
Try out a few different titles. Keep them in mind as you re-read your story. Choose the title that deepens in significance--becoming more ironic, humorous, tragic or haunting--as the story develops. Shirley Jackson's "The Lottery" is about a lottery, but one that is surprising and disturbing, not revealed until the end.
Test your titles on your friends or select readers. Ask them, "What does this title evoke in your mind?" Your titles shouldn't be too obvious or to the point, but they shouldn't be too tangential either.