This is an explanation from Gitesh Pandya (Box Office Guru) on how marketing strategy for Slumdog Millionaire:

Slumdog Millionaire was initially released in only six major cities on November 12 and American and desi moviegoers alike were targeted.  I invited the South Asian media to come and see the film early as we screened the film over a month before its release date so journalists could do stories on it in order to raise awareness.  The film was selling out at all theaters and each week more theaters were added in more cities across the country.  The momentum kept building and so before Christmas Fox Searchlight expanded the film to over 600 theaters in big and small cities alike.  It continued to increase its weekend gross every week as people have been recommending it to others making it a true crossover success.

This has been an increasingly effective marketing strategy. Here is an account of recent developments from The New York Times:

“Films Reach Theaters a Drib Here, Drab There.”