This is the holy grail of posting strategy - reaching more people than your follower count. And we have a couple of tips to share that we have used in the past to increase the reach of your posts without using the "Boost" option, which costs you money.
First, pay attention to the time of day you are posting, and how many people are reached. For one of our clients we noticed that if we posted on a weekday afternoon we reached more than 100% more people than if we posted in the morning. Our theory is that their followers are more likely to read and comment on our posts, and more likely to share them with their friends, in the afternoon. A simple thing like the time you post can make a huge difference in people reached.
Second, post items that are of interest to your follwers. While your message is important, if you're posting things your followers are not interested in reading and reposting, you will be more likely to reach less people. Pay attention to which kinds of posts garner the most responses and the most people reached, and tailor your posts to meet the expectations of your audience.
The bottom line is this: If you increase your reach, your message will reach more people outside of your follower circle. And as you reach more people, you will ultimately gain more followers, who are then more likely to become a member or client for your organization. That should be the ultimate goal of your social media strategy.