From the Search Engine Optimization FAQ:
I just changed my domain name. How can I switch without losing my rankings?
You’ll need to do a 301 redirect from the old domain to the new domain. Fortunately this is not difficult to do. You’ll need to add the following lines of code to a file called .htaccess and place it in the root directory of the old domain:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www\\.)?old-domain\\.com$ [NC]
RewriteRule ^(.*)$ http://www.new-domain.com/$1 [R=301,L]