To mark all posts as already seen, click the "Quick Links" at the top right, then click "Mark Forums Read" in the drop-down menu.
Also, don't forget, if there are some sections of the forum that you're not interested in, for example "International affairs" or the "Marketplace" stuff, you can go to "My Settings" and then "
Edit Options", then scroll to the bottom and select forum sections that you wish to exclude from the new posts list.