1. popular
2. new
3. popular
4. new
This injects a lot of noise into the front page, because most of new is noise, but it helps.
Another option is to front-load popular stories, then have 10-20 "new" items at the bottom of the page. That way the front page remains highly salient, but anyone who merely scrolls more will also see a small chunk of new. So 1-30 popular (as it is now), then 30-45 new (making the frontpage longer).