There are a couple of ways. There is a little button just above the topic that is called "bookmark" click it and the topic will be added to your bookmarks. You can give it your own title so it will be easy for you to remember why you bookmarked.
You can also post a message to the topic. Any time someone else posts a message it will show up under Watched Topics for you. If no one else posts, you can find it by clicking "My Posts" and find your post. That can get cumbersome though if you post a lot (like me!)