Something that happens far too often on a site is that the site manager cannot help but tinker with the site structure. Tweaking a site from time to time is a necessity, but it can go too far. As Google is king when it comes to assessing page rank, it pays to know a little bit how Google works and why change is not always a good thing.
When content is added to a site, or a page is changed for that matter, Google re-indexes the site. This is an automatic function that is carried out, and while sometimes change can bump your ranking up, it can also drop your rank or in rare cases leave it unchanged. Why does this happen? For starters, when you add content you change your keyword density. Keywords you were previously very strong on might take a small downward, but it stands to reason another may tick upward slightly. Even if you change the actual design of your site on a regular basis your page rank may suffer.
Take for instance the changes Google has recently made to their algorithm in which sites that flood out large quantities of content on a regular basis have taken massive page rank hits. This is especially problematic for community contributor driven sites in which many contributors not only do not understand SEO, they may not even care. As hundreds or even thousands of submissions come in each day, every calculation regarding page rank is reworked putting the site manager/owner in a position in which they never know where they really stand.
Is there any way to retain or improve a strong page rank while still adding new content? Fortunately the answer is yes. One key is to keep the original content of the site as in tact as possible. If the original content was good enough to start with there is no need to cull it and lose the positive benefits it is providing. Weak content that has poor keyword density can and should be culled, but not in a mass exodus manner. Removing weak content bit by bit is the best way to go. Think in terms of adding two strong SEO pieces of content before removing one weak piece of content.
Furthermore, add content at a reasonable rate. In most cases, adding one new post per day is as much as most blogs/sites needs. Adding ten new posts one day and then none for several days does you no favors when it comes to the distribution of page rank. If you do however decide to give your site a major makeover, consider shutting your site down for a day or two. When you go live with your site after optimizing it in this manner it is akin to a reboot. The key is, as always, start with quality and add quality – and always keep the best principles of SEO in mind.