There isn't really a simple answer to this because the publicity needs of every film are completely different. You promote a $150m film starring Tom Cruise in a completely different way to a $15K indie feature.

At the $15K end, you need to have a pretty amazingly good film to have even a slight chance of success. The best bet is to try and get the film into one or more medium to high profile film festivals, engage the services of a professional publicist (they are not always as expensive as you'd think - particularly if they like your film), and formulate a proper sales stragegy to get a distributor interested. Once a distributor(s) is on board, it's largely out of your hands.

Ben.

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And, for that matter, think about how you're going to sell it before you shoot it: 'Blair Witch' is a good example, being successful almost entirely due to the marketing campaign they came up with, not due to the movie itself.

An awful lot of movies get made because the director/producer/writer has a great idea they want to make, but never get anywhere because no-one else in the world thinks it's as great as they did, or it's not great enough to compensate for the limitations of the budget.

Thank you for your answers, I'm looking for more information about the history of film promotion. Basically, the big movies history. I haven't found any web sites describing examples of promotion campaigns in details with professional comments.
Maybe some stories like the one about the 'Blair Witch' would be interesting to me. I'm sure it's not the only successful PR and marketing campaign in the film industry.