Whether you’re making a commercial, or a feature-length movie, you have a chance to tell people about it before your release. I’ve seen many different methods, but the most effective is creating buzz by giving out teasers of information before the release. Some producers give out lots of information, some don’t give any.
One school of thought is keeping a lid on your project until it is complete. This keeps any imitators at bay from trying to copy your ideas. Also, if something happens and you can’t finish your film, you don’t have to give the appearance of being a noncompletionist.
The other school of thought is what you will typically see in a large-scale production. Most productions like this will start issuing press releases as soon as they have a script or cast. Then issue more press releases with each new development. Yes, there may be copycats, but people will start talking about the project, and it will get people talking and generate buzz.
The best course of action I suggest is a compromise between the two. Don’t try to keep anyone from talking about your film or posting pictures about it. However, don’t tell the entire story of your film before you release it. Keep this balance and you’ll be able to promote your film without going overboard.