I think if you try and you keep to forums that are related to the topic or what you are selling forums can produce leads. Maybe one of the better choices for free marketing.

     I think having a good heading , headline and clear discretion can good a long way and you may see some action on your visitors pages if you hit the right people. Try not to have to many links and focus one one topic at a time. also if you do get leads you can try your next promotions in a week or two that way your not always posting a new thing were people stop looking at them.

    When choosing a forum I would use a search engine than what the product is and forum to find places you might want to post. A catchy banner I find sometimes helps.