Know your audience
If you want to reach out effectively to potential clients, you must know your clients and their wants/needs. You may accomplish this by "listening" to their posts, tweets, questions, etc.
Stay focused, and be the expert
A few off-topic posts are okay for generating conversation, if needed; however, do your best to make sure the majority contain relevant and expert advice. Your clients and future clients are coming to you because you have needed knowledge.
Your client feels like #1
By taking the time to listen to your client prior to posting, you have just made that client feel as if he or she is your only client. You took the time to read the post, do some research, and reply with helpful advice and/or a useful link. You took time, and this is plain to see because you answered thoughtfully.
Always remember that good listening is active, not passive. This means that you must take the time to understand, interpret, count, and evaluate what you are "hearing" from your clients. Do this, and you will have successfully listened to your clients, generating a positive outcome.
Engage your clients on all of your social media levels. Ask yourself if you are sending a message that reflects you and your practice, and if it reflects the client's interest as well. Listen to what your clients are saying, and reap the benefits.