Find a mod that allows you to modify member and user online counts to create the "Sense" of a liked forum. If people see ZERO people on the forum all the time or 3 for that matter odds are they will know it is dead there. I don't personally think faking anything is a great business model to follow BUT it would work.
Update your forum with every bit of sports news you hear or read and post and the results and fixtures etc, then post around saying what you have on your forum
Content and controversy. Appeal to the fanatics with discussions about various games, league rules, player stats. If you have comments enabled on your blog you may wish to disable them and have at the bottom of each post - "discuss this further" in a link that takes you to the forum. You would want to make a thread for each blog post you do. Do not copy the full blog post into your forum - just link back to it. You don't want to have duplicate content on your site. Give that a try.