301 Permenant Redirect

I find it funny that when setting up new wordpress blogs, it automatically sets the address of the website as a no-www domain, however i am having problems changing some of my domains from no-www domains to www domains. Any suggestions? How can I do it after my sites are live? Also can I do this through cpanel, I have tried this code in the htaccess file

RewriteEngine On
RewriteCond %{HTTP_HOST} ^yourdomain.com$ [NC]
RewriteRule ^(.*)$ http://www.yourdomain.com/$1 [R=301,L]

but nothing seems to happen. I am already starting to notice search engines thinking my addresses are two different domains. I even have a different number of backlinks for each variation of the domains? Help anyone? Thanks in advance.

cpanel has redirect options...that's how i do my 301's. It's kind of tricky with wp blogs cause they will cause redirect loops sometimes. If it's already live i'd just leave it without.

Yeah that's exactly the problem, I get loop errors.