Ideas for marketing your film need to begin in the initial development phase. From the first read, you should have an idea of who your audience will be. Will it have mass appeal or is it more for a niche market? From its inception, a script is usually typecast in a certain genre and then age group and even by sex, i.e. thriller for 18- to 30-year-old men . By honing in on a specific audience, you will be able to ensure the development, creation and exploitation of the film is capitalizing on your audience's desires, which will translate to greater success at the box office.
Once I have my marketing plan, I lay it out in a business plan for my investors. Even if you aren't planning to devise your own business plan and approach private equity investors, you will need a plan when you pitch the project to other producers, studios or financiers. They want to know your project is geared toward an audience they service.
If it's not clear who your audience will be then your script probably isn't focused enough. Spend time developing the story so it targets a certain kind of audience. Whether it's male, female, young, old, indie, mass, etc. Have a strong idea of your audience and what they like and build a marketing plan for targeting them. Every film is not for every person. Your audience is built of individuals with specific tastes. You want to harness the group with the taste you are trying to reach.