Useful SEO techniques when re-building a web site

Over the past few years, we have taken over the development and maintenance of several well established high profile websites and more often than not, we've been changing the technology that the sites are powered by to our preferred solutions. One of the major challenges along the way especially with high traffic sites with thousands of back links is to try to preserve those links whilst deploying a new site that may or may not have the equivalent pages and or structure.

So what tools and techniques are available to us?