The most important thing is not dissimilar from conventional marketing. It's important to have an understanding of what your audience wants and how you can supply it.
Using 2-way channels like blogs or social networks, you can try to develop a conversation about your product. Find out where your potential customers hang out and engage with them. Comment in blogs or social networks, send messages, create videos and other valuable content and send trackbacks.
What Christian said is spot on. Your message has to make people feel they are benefiting from the engagement and they aren't being sold something.
