You can also go to the thread, and when looking at the first post at the top of the page, you see the "post reply" button. Look over towards the right, and you'll see the worlds Thread Tools with a downward arrow. Click on "Thread Tools" and a "dropdown box" will appear, and in the list you'll see "subscribe to thread".
Click that, and it will take you to a page where you will see "Subscribe to Thread" and the name of the thread after that. There are dropdown boxes below, and you can choose if you want instant notification, no notification, or daily or weekly updates by email. Choose one, and click on "Add Subscription", and it will take you the beginning of the thread.
And that's how you can subscribe to (or unsubscribe from) a thread without posting something!