When it comes to marketing it can be a bit hit or miss. There are things people say work but not always while other things work for others that don't work for others. So I went with the theory of you can knock it until you try it.
Step One: Research. and then some more research. I've made films before so I know the basics but none of them have been very good and it has been a few years and technology really has come a long way in that time. I first needed to know what others were doing. Most I found were really just bad. Mostly because they missed the point. You are trying to sell your books. Instead, a lot of authors were too wordy (surprise, surprise) and they failed to engage their audience. Certainly I didn't feel the urge to rush off and buy any of their books.
Step Two: More research. Not to be confused with the previous research I spent most of my time trying to get what I wanted to work. Actually didn't get that far as to be honest to make what I wanted you needed better software than what had. Like what the heck SWF's are and why movie maker doesn't like flash and all the adobe programs that even though I'm familiar with photoshop confused the heck out of me. Still not completely happy with the final product but after a few frustrating hours I kind of gave up.
Step Three: After all the research I now knew I needed a tension filled audience gripping script. Images that were dynamic enough to portray the ideas in my novel and do them justice. And everything had to be under a minute. 30's would have been better but I found my script was a bit long. Throw them all into movie maker after spending way too much time trying to get the words to work in Photoshop animation. (since I was familiar with the program it is actually quicker than trying to learn something new) I then explored the wonderful world of the creative commons for some music. Rather easy compared to putting the images together to be honest and tada like magic a video.
Step Four: Now this is the part everyone forgets to do and that is me included. You have to tell people about the trailer. Most trailers I see on youtube are maybe seen two dozen times at most. My first step is to share it on social media. Then on my site and then put it on other sites that sell space for ads. Then I think I might capitulate and put it on youtube as an ad for another clip that is getting views but that is very, very expensive and will require a lot more research. I think if I do that for one of my books I might splurge on that fancy software I was dreaming of while I swore at the short comings of adobe's plethora of chunky programs."
I've put the video on here but I have the feeling I might change it still. So this might be different the next time you look