To find an old post, or to follow one that you've read, look under the original post for a box that says "follow discussion." The default is "no" so change that to "yes." The post will now show up under the Watched tab along the top of the discussion threads. To unfollow a thread just change the box back to "no."
You can also check the box under the reply window that says "I want to follow this discussion on my discussions tab" when you post a reply, but I usually just change the Follow Discussion box. I think it only works if you actually post a reply and most of the time I forget to check the box. LOL