A blog is a great way to promote your film, both before and after it's made. During production you can keep a diary of each day's work on the film. Afterwards you can use it to promote special events in the life of the film -- the festival submission process, upcoming screenings, other work by the cast and crew, and (for documentaries) updates on the film's subjects. People always want to know "what next?" and "what happened to so-and-so?" Let your blog be the delivery mechanism.
There are lots of ways to get started on a blog and I have lots of opinions on the subject (naturally), but for now I'm going to refer you to the Caffeinated Librarian, who has put together a good set of resources for first-time bloggers.
If you've been blogging to promote your film, feel free to let me know by e-mail (chris at stomptokyo dot com) or in the comments. In a future post I'll link to some of the best film blogs I encounter.