Reverse prospect the building owner and target your message to their needs; move up down size if your unit fits that criteria. Look for recent sales in the building, where did the buyers come from, target those areas. Circle prospect the neighborhood. Call the people you know and see if they are aware of anyone with a need for what you have. The closer to face to face you can get the better but don't forget the power of networking sites, photos and video. That should get you started.