1) Find the appropriate forums that correspond with your niche and contains your target audience.
2) Find threads where people are asking for help regarding your niche.
3) Politely reply them in the thread only (Do not just ask them to go to your site). Follow the rules of the specific forum and respond to the thread or use signature link to let them visit your site naturally.
4) Have a presence and be helpful. Nobody responds to a spammer or a pushy sales person.