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Discussion groups or forums are places where people get
together and talk about a common interest. If you sell
bodybuilding products, you may want to get involved in
bodybuilding and weight lifting forums. Most of these
forums have strict regulations against advertising and spam.
Be sure to read, then obey their guidelines. 
Become part of the group. Participate in discussions with
credible information and opinions. Don’t participate in
heated debates or mudslinging. What you want to do is
build a reputation for being someone they like and trust.