You have to follow the below steps
http://codex.wordpress.org/Changing_The_Site_URL (Use the Redirect Method in this link)
http://yoast.com/move-wordpress-blog-domain-10-steps/
Inform Google via webmasters tool
Update Analytics Code (http://www.koozai.com/blog/search-marketing/how-to-change-website-domain-in-google-analytics-9918/)