Have you ever played "feather blow"?  Take a small, downy feather and whoever can keep the feather in the air the longest (using only his/her breath) is the winner? Well, new content on Google is a bit like this.

New content from an existing and established web site has a tremendous opportunity to get search rankings relatively quickly - especially for long tail keywords - but these pages suffer from "Google Gravity" just as the feather suffers from ... uh, "real" gravity.

When you first release some new content such as a post on your blog, Google takes it and puts it on the "fresh" list.  You will gain some decent rankings - especially with less competitive / long-tail keywords.  But it won't take more than a few weeks before this content starts to slide down the rankings ("Google Gravity"), just as the feather will soon float to the ground.

Once you have your content created, the key to keeping it in the top of the SERPs is to make sure that people are linking to it.  This is one of the ways to get it out of the supplemental index and to stabilise your search engine rankings. 

There are many ways to do this of course, but here's a few quick ones:

1. Reference previous, relevant blog posts in the copy of new posts as mentioned in Pete's recent post - SEO Blogging Tips - Link to other posts on your Blog  

2. Use social media sites such as digg and stumbleUpon and get a number of people to start tagging your new page/post

3. If it is newsworthy or a bit controversial try to get a few bloggers to discuss your views (with appropriate references to your new post).

New content and especially blogs can gain rankings very quickly and can be a great tool for increasing your search engine saturation.  If you want to keep the rankings though, you need to make sure that your content gains some credibility online with inbound links (both internal and external).