To suggest your post is a little to broad would be a ridiculous understatement. What niche are you targeting? One affiliate offer may be great for site A but terrible for site B. More information required... WebDev
You will have to test several inside your niche. If it's not in the niche of your forum or at least somewhat related, you probably won't have much success with it. Also if you're getting that much traffic some CPM ads might be effective