Click on the globe to the immediate left of the thread topics. If you click on that, then all of the posts in the thread, will automatically be marked as read.
I just discovered the 'Mark All Forums Read' link near the bottom of the main forum page. It's a useful way to mark everything read, especially for someone new to the forums like me who doesn't want to see all 9,806 posts as unread, just the new ones since I signed up.
