Once you have checked the forums that you are interested in, click on Actions and select Bookmark.
Notice that you will then have a bookmark tab at the header of the site. When you are surfing and there are additional postings in those discussions, a number will appear above the Bookmarks tab.
If you have also set up email notification for the bookmarks, then an email will also be sent to your email address. To activate email notification, click on Profile in the top header. And then select the email Notification tab for : when a bookmark is updated.