Your platform is the combination (hopefully) of ways you have developed to reach and cultivate your potential audience/customers as well as maintain relationships with the audience/customers you already have.
First-time authors cannot underestimate the importance of platform, as many a well-written book has been passed on because of the author's lack of platform. Meanwhile, plenty of decent, but not great books have been published (and their authors have received huge advances) simply because the authors demonstrated their skill at developing and quantifying their audience. It might not be fair, but it's the way it is and there's no use complaining about it.
What can you do to develop your platform?
- Create an interactive website and blog, give away some free advice or free products, and also sell products online if that's what type of business you have.
- Write articles for magazines and popular websites and blogs that reach your target market. If you'll only write if you get paid (or paid well), you won't be doing this work because it either doesn't pay at all or doesn't pay well for people who don't already have an audience. Don't forget to link whatever you write elsewhere to your website and blog.
- Do a podcast. And if your audience is particularly into/dependent on podcasts, this should be a requirement.
- Give seminars or workshops on your topic. At first, you'll be working for no pay, but that should change. Remember to refer every audience you speak to to your website and blog. You are in charge of your own promotion; make the most of it!
If you're attached to the idea that you're an expert and should always be paid for what you do, I suggest you detach yourself as soon as possible. No one wants to work with a resentful diva-type--particularly if that person isn't already rich and famous.
Furthermore, no matter what you're doing, think of a way to promote or cross-promote it. Every public appearance should be publicized, as should every guest appearance on a blog or podcast.
Finally, give your audience the opportunity to subscribe to tips or a newsletter or something you'll develop to grab their e-mail addresses so you can keep in touch with them. Most people aren't going to reach out to you unless you make it easy for them. They'll remember you made it easy for them, and you're on your way to amassing (and quantifying) your audience before your book reaches the shelves.