there are many way to get backlink, such as directory submission, forum posting, article submission, social bookmarking, guest blogging, link exchange, link purchase etc
Forum links can help drive traffic through direct referrals in addition to SEO. It helps to write thoughtful, intelligent comments that add value to the discussion.
Use do-follow social-Bookmarking websites, write blog with informative content, article submission and blog comments( Do-Follow). You can use free classifieds ads as well.
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Forum posting may a good tactic for a forum, as you are tapping into an audience that either regularly browses forums. Blog commenting would be another good way, posting comments on blogs with relevant content and adding to the discussion is always a good way to build backlinks. Guest blogging and link exchange would be a good idea too.
there are a lot on the web that offers posting free classified ads. but be careful of choosing the "real free ads posting" because some of them are just frauds ! goodluck
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