A trick I'd do is host a trailer on your site then see who downloads after watching it, or how many more things they look at or how long they stay looking at stuff.
Interpreting the numbers would be a little subjective, because you can't release trailers too often, and there are other factors with each release that are hard to account for, such as the type of people entering the site and so on.
A possibly dirty trick... make 3 trailers that represent your 3 best attempts to make a good trailer. Each trailer should follow a distinct philosophy about how your trailer should be built. Then you randomly serve them up when people come to your site. Then you can really tell which ones are preferred.
You don't have to lie about it. You could serve them up randomly, then underneath explain that there are others that can be viewed. People won't care that much. If all 3 are good, and legitimately represent what you think is a good trailer, then people will get a good trailer. And you will learn a lot.
Just make sure your site is set up well first. You can use this same tactic for learning about the best way to present all information, screen-shots, extra info, wording, whatever.
Alpha builds are acceptable because you need the feedback to make the game better. The above is like an "alpha press release."
I'm liking this idea.