1) Your story needs to have some kind of aspirational quality. It needs to be meaningful. If your story is violent and really a downer, it's not going to be enough of a draw for people to follow it across multiple platforms. There's an upbeat quality to most successful transmedia stories. It's got to be a story world you want to spend time in.
2) You have to understand the media platforms that you will have at hand. If you're an independent creator with a little budget, you still have access to social media, the web, independent digital publishing. You need to understand the language of each of those platforms. If you don't want to, find someone who can help you understand the strengths and weakness of each of those platforms. You need to be able to design your story to play to those strengths and avoid those weaknesses. You're already developing a design sensibility for the media platforms you have at hand which will help you develop the story and ultimately, produce it.
3) You need to think about and ultimate build an architecture for dialogue around your transmedia implementation. This, in essence, gives your audience the ability to provide your audience with feedback. You can make it fancy and have that become a part of the narrative if you want where your characters can literally communicate with the audience, but that's not a requirement and it's tricky to do that. We now have the ability as storytellers to look into the eyes of our audience and to validate their participation -- by which I mean the audience has a need to express themselves - from their opinions about what it is they're experiencing with your story to creative content and story-driven user-generated content. They're doing more and more of this in social media. It is my belief that if you ignore them, they'll go away.