I can't stress enough just how important your film's trailer is - especially that very first trailer you release publicly, which could very well make or break your film. It's THAT important; it's your film's initial introduction to the world, and you know what they say about first impressions.
Given what I do here, I get emails daily from filmmakers alerting me to their projects (whether features, shorts, web series, etc), and I'm continuously surprised at just how seemingly slapdash some of the trailers that accompany those email alerts are, which is unfortunate, because there actually might be a damn good film being summarized in that 2-minute clip. BUT because it doesn't appear to have been very well thought through, and thus not very well put together, it immediately sours any interest one might have had in wanting to see the film in full.
I don't want to mortify anyone by using their trailer as an example of what NOT to do; So, instead, I'd just say that, when cutting a trailer for your project, take these 6 suggestions into consideration:
1 - How well do you know your film? You really should, by the time you get to putting a trailer together. What's its center? It's core? What's the thread driving its narrative? Think about that as you nip and tuck, and make sure it comes through clearly by the time you're done, by getting to the point of it all.
2 - Who's your audience? Who are you targeting with the trailer and ultimately the film? And what do you want them to think and feel after they watch it? Testing the trailer with a select group of members from your target audience, before releasing your trailer wide, can be a good thing!
3 - Limit the music. In my experience, too often, there seems to be this need to mimic Hollywood's approach, with timed, swelling soundtracks that sometimes overpower the images and words. That shouldn't happen; the music certainly shouldn't be at the same sound levels as the dialogue within the scenes; I shouldn't be struggling to understand what's being said because the spoken words are competing for attention with the music. Let your scenes (the actors, dialogue, cinematography, production design, direction) speak for themselves. The music shouldn't do that for you. It's there to compliment.
4 - There's no such thing as the perfect trailer. I'm sure you could always find something to critique in any trailer; but if there was one single suggestion I could make for you to take from this, it would be to K.I.S.S. Less really can be more. Less of everything; don't give the entire movie away; some mystery is a good thing. Keep the music to a minimum; keep the trailer short - I'd say from 1:30 to 2 minutes is ideal, although you could make it even shorter. A powerful 30-second teaser can do so much more than an overstuffed 2-minute meal. The trailer is meant to intro your film, and, in effect, tease your audience into wanting to see more of it. Then after you've released your first trailer, you can release additional teases with more clips, stills, etc.
5 - Watch trailers for other films, especially films that have already entered the markeplace. Of course, it's all subjective. What works for me doesn't work for everyone else. So identify those trailers that worked for you, meaning that they got your attention and made you want to see more of the film. Watch as many of them as possible. Break them down sequence by sequence if you have to. And then watch your trailer for comparison.
6 - Take your time! Don't rush! As a filmmaker myself, I know the excitement one feels after completing a project and wanting to share glimpses of it with the world soon thereafter. Patience is important. Just as you take your time in writing the script (1st draft, 2nd, 3rd, 6th, 10th, etc), do the same in cutting the first trailer taking everything I said above (and more) into consideration.
Obviously, if your original source - your footage (the acting, directing, cinematography, etc) - isn't great, then you really can't expect to be able to cut a great trailer. However, I think you all will agree that there have been those times when you've been fooled into watching a film because you were sold on the trailer; Meaning, even with weak source material, you can still put together a decent intro to your film in the form of a trailer. Hollywood studio marketing departments do it all the time.
If you need assistance in putting your trailer together, please don't hesitate to ask. Not that I'm some trailer professional; I just know what works for me, so you'd be getting my own personal suggestions, and you can do with them as you see fit. Like I said, it's all subjective. But even if you don't ask me, ask others whose opinions you respect - and not just your family, friends, crew or cast mates.
To reiterate, your first trailer is your film's first real introduction to the world, and it could impact how your film is viewed from then on, even if you cut a second improved trailer. That initial sourness really does linger on for awhile, so take that into consideration before you hit the "publish" or "send" button.
Filmmaking is already hard, time-consuming work; don't ruin your film's potential reach with a weak first trailer.
Of course, this is all one person's opinion, based on all I've seen and heard, after watching numerous trailers, on a daily basis, over the years.