A trailer is the best way to market your film, but you have to do it right to attract an audience.
Let Them Know
Self promotion and advertising is important to independent film even more than studio productions because you do not have the resources to announce its existence. With this you need to be extra creative in the conventional realms to let people know about your work. The trailer, known as the preview to audiences, is the standard way of advertising your film. This acts as a commercial for your movie that tells a little about the film, using footage from that film. There tends to be similar trends within trailer construction and there are a few things to keep in mind to help you put together an effective one.
The main thing you need to think about is to create a narrative structure with your trailer. This does not mean that you need to place the entire story into the trailer, but you at least need to set it up and take the audience into part of the rising action and climax. The majority of the trailer is taken from the first act of the film, and then the rest is a collage of the middle acts. Select clips that will identify the main characters and conflicts, and then begin selecting shorter clips that will then identify some of the more dramatic or comedic moments of the primary action. Do not give anything away in the trailer that you would not want the audience to know when going in.
You may want to use the trailer for a more visceral, rather than intellectual, experience. This means that you would prefer the audience to not know much about the actual storyline and instead use music and fast cutting to elicit an emotional response. You still should try to maintain some type of linear storytelling model, but it may be a good idea to focus on music rather than actual film audio during this type of trailer. If your film is more character oriented and less concerned with visuals then this may not work as well.
The length of the trailer is also important, and making it too long is a common mistake. Conventionally a short film trailer should not go over thirty seconds and a feature should be around a minute and a half. This is not a finite ruling, but should be thought about when you are putting it together. The pace of the trailer is the most important thing to consider, and if you cannot keep this up during the course of it then it must be shortened.
Consider the clips that are going to make your target audience want to see the film when selecting them. This does not necessarily mean the ones you like the most because the audience is only going to get a brief image of them. Try to get ones where the dialogue or imaging will tell the audience quite a bit in only a matter of seconds. You do not have them for long so you have to maximize the time you have.