Sometimes, if you use a CMS like WordPress or Movable Type, you could need to change the URL structure in use on your blog with a more SEO friendly structure.
This is a big problem especially if your site is already indexed and some posts are popular on search engines: if you change the URL structure, all your posts will be unavailable at the old URLs and, in this way, you risk to lose visitors and ranking. A safe way to change the URL structure of your blog, without losing visitors and ranking, is to apply a permanent redirect to your pages (301 redirect). In this post I’m going to illustrate how to apply a permanent redirect to your pages using some techniques and scripting languages.

More at: http://feedproxy.google.com/~r/Woork/~3/gOHd6aEFYvQ/