In a thread-- just between the thread's posts and the compose box is a blue bar, and in it is the phrase ADD TO TRACER. Just click it.
tHE WORD trace ALSO APPEARS IN THE FAR RIGHT COLUMN OF THE TOPIC LIST, AND CLICKING THAT DOES THE SAME THING.

iN EITHER CASE, THE THREAD TITLE (oh DAMN capslock!) appears as a link on your personal page, along with the color change when a new post has been added.

Of course, you also can bookmark any thread in your browser or find it in your beoswer's history file.

~Susan