A great film to show us the power of teaser trailers was Cloverfeild, a 2008 Science-Fiction film that released an initial teaser poster, full of hints about the movie, but no film title - this was a clever campaign that sparked interest.
Most teaser posters aren't as mysterious, as conventionally they include the title, but no actual release date - just a month, or often a season, such as 'Autumn 2010', or 'Fall 2010' in The US. Other conventions that are normally included are a large image to capture the audiences attention, yet relatively plain overall, as not to give too much away about the film.
