In the past there was a functionality to add threads to your favourites which could be viewed from your profile. This feature was discontinued with
the recent upgrade - as far as I know. So the best way to keep tabs on your favourite threads is to post in them.
Actually there is a "Subscribe" button underneath the Flag button (in threads you haven't posted in). This will add the thread to your Subcribed
threads (visible under myATS) without having to post in it.