Off-page optimization like forum posting, blog commenting, directory submission, social bookmarking and article submission will help to get more and more inorganic traffic.
Thats great that you are getting good organic traffic which is he main source of traffic. For increasing the inorganic traffic, do social networking, bookmarking, forum posting, PPC campaign.
U mean the natural traffic. You need to optimize your website using seo or sem methods. Start with any like
1) Write meta tags for your site
2) Write good content to your site and check the keyword density for seo.