Forum normally has different layout as compared to normal websites. Forum visitors are also likely to be repeat visitors, which over time could have developed ad-blindness; the predictable nature of the ads make the visitors (maybe unconsciously) ignore the ads.
The tips below will complement the AdSense unit placement strategy so repeat visitors will notice the ads more.
Forum regulars are so used to the site that their eyes and attention goes straight to the first post. This would make placing your ad unit around the first post to be quite effective.
With ads relevant to the site's content served via the AdSense program, ad unit placed immediately after last post will answer the visitor's “what to do next” question.
Placing after the footer would not be a good idea as visitors who go the next thread will not see the ad.