if you want to do it right and not get caught use fantomaster.
if you want to it using mod rewrite with the chance of getting caught (and banned from the SE's all together) just add this.
RewriteCond %{HTTP_USER_AGENT} !^.*googlebot.*$ [NC]
RewriteCond %{HTTP_USER_AGENT} !^.*slurp.*$ [NC]
RewriteCond %{HTTP_USER_AGENT} !^.*inktomisearch.*$ [NC]
RewriteCond %{HTTP_USER_AGENT} !^.*scooter.*$
# for each search engine crawlser you'll need to add the .*spider-name.* Note that there are hundreds of active crawlers.
RewriteRule ^.*$
http://www.newdomain.com/ [R]
then add this meta tag to all the pages that are cloaked <meta name="robots" content="NOINDEX">
anyways if your site gets banned I warned ya