To maintain more stable ranking, you really do need to keep adding new information and even update the same few pages on your website often, like your home page, events, and special offers. An easy page to update monthly is your "birthstone of the month" page, just be creative and tell people about popular birthstone jewelry and how to match it with clothing fashions.
Google uses a very fast method to check the last update time of one of your web pages. It does this instead of spending the time to re-read your whole page. If they find a new page timestamp they send the Google spider to fetch the page.