I'm not sure exactly what you mean. But here's a run down.
There's two ways you can go about this. When you're in the 'new posts since last visit' page, it's probably best to view all the threads in new tabs. So when you're done in a thread, you just close the tab and the 'new posts since last visit' page is still there. Or if you don't like using tabs, the search bar on the top right of any page of the forum can take you back to the 'new posts since last visit' page.