If you want to increase the referral/word-of-mouth traffic to your community, there are a number of tactics you can use.
However, it's important to understand the basic principles.
First, word-of-mouth/referral growth is the best type of growth for a mature community. It replenishes the members you're losing, attracts the members that are most likely to become regulars, and broadly helps foster a stronger community (newcomers already know people).
Second, people refer others/share content for one of two reasons. They either want to impress people (increase their status) or help people. The former is typically stronger than the latter.
Your tactics to encourage WOM must live along one of these two axis; increase status or helping others.
Let's take that first axis; increase status. You could, for example, take the concept of involvement. If you're involved in the co-creation of a community eBook, you're more likely to share it. It increases your status amongst the people you're sharing it with.
Likewise, if I can invite people to exclusive community events/activities taking place, that increases my status when I share it. Status-orientated tactics work well.
Along the second axis, helping people, it's key to realize these people have less knowledge than your members. Be sure to appeal to more simple questions about the topic, newcomer guides, how to get started, basic equipment advice, newcomer bonuses etc..
Too many communities hope for referral/WOM growth to happen as they grow. That's a mistake. You need to master the principles and proactively drive it.
P.S. Join me for a free webinar, Unleashing the ROI of Online Customer Communities on 24th January 2013.