At the top of every page of every thread is a button labeled "thread tools." Clicking on it generates a drop down menu. The bottom of that menu is a button to click to subscribe to that thread. What is the deal with you people who use forum posts to subscribe to a thread? Are you seriously subscribing to every thread in which you post? Don't you drop in on some threads just to be a wise ass? They're not all worth revisiting!
In the image below, the button is giving me the option to unsubscribe from this thread. I subscribed via thread tools when I first visited it.