Keep page names static. Once you have created them, leave their page names as is, giving search engines enough time to index them and time for Inbound Links to accumulate.

If you look at how most newspapers build their websites, you will see a good example of this in action. Here is an example:
http://www.guardian.co.uk/world/gallery/2008/may/30/brazil.photography

The URL is constructed to preclude the possibility of a duplicate URL being used in the future. As the Guardian newspaper accumulates more and more news items, the sheer bulk of their content, as well as the persistence of their pages, makes their website very attractive to search engines. They have over a million Inbound Links to their website. No surprise, then, that the Google PageRank for their home page is a whopping 9 out of 10.

A Content Management System (like Big Medium from GlobalMoxie) is a great way to keep your content indexable by Google.

If you change the URLs of your web pages as you change content, make sure you keep the older pages, and also that they are linked to from within your website so that search engines can find them.