Trailer
I work in Film and Television Advertising and I have to say the obvious, your trailer is the most important thing to getting people interested in your film. I've worked on dozens of movies that were less than stellar and there is a lot you can do to make a bad movie look good. However, the inverse is also true. You can easily do yourself in with a bad trailer. You need to divorce yourself from the film and look at from a marketing stand point. Often times less is more. Leave the audience curious to see more of your film. SHow the best parts! Also, never use anything that makes your film look like low production value film. (unless of course that look was intended in which case they better get that too). Make sure your stars (if any) make it into the trailer no matter what they mean to the plot. Also, remember the viewer needs to come away with an understanding of what this movie is about. It may seem obvious but far too often trailers come across as overly complex, in terms of plot. The trailer should really follow a three act structure just as your film hopefully does. Although slightly different, it's set-up/ inciting incident 'the hook', then what are we gonna do about it/ journey begins, third is usually raise the stakes and cue all the fun and games montage bringing us out on a high note. Also, unfortunately often times once a film is done you realize it's not exactly a genre most people want to see, but that doesn't mean you can't sell it as a genre that is popular. The idea is getting butts in the seats so they can see your brilliant your film. We've all seen trailers and realized after how it was completely different( for better or worse, it can be frustrating) but the marketing did it's job! That's just a little of what I can think of off the top of my head. Every movie is different.