Probably the easiest thing is to bookmark the topics you may want to come back to.
When you have the topic open, use the Browser's "Add to Favorites..." or "Bookmark this page..." command, and a link will be saved, with the subject heading as its name. If you want to flag a specific post within a long topic, the heading of each message (e.g. "Re: flagging messages") is a link that you can bookmark - just right-click the heading and select the command to "Add to Favorites..." or "Bookmark this link..." or similar.
Of course, you can also use Search to find something in the previous posts.