The Google Webmaster Central Blog has posted about “Planning on moving your site to a new domain?” — best practices to follow when moving your site from one domain to another without hurting your site's performance & rankings in Google search results.
Your aim is to make the transition invisible and seamless to the user, and to make sure that Google knows that your new pages should get the same quality signals as the pages on your own site. When you're moving your site, pesky 404 (File Not Found) errors can harm the user experience and negatively impact your site's performance in Google search results.
Let's cover moving your site to a new domain (for instance, changing from www.example.com to www.example.org). This is different from moving to a new IP address; read this post for more information on that.