If you want to redirect all non-www requests to your site to the www version, all you need to do is add the following code in your .htaccess file
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
This will redirect any requests to http://mysite.com to http://www.mysite.com. There are benefits from doing that:
* It will avoid duplicate content in Google
* It will avoid the possibility of split page rank and/or split link popularity (inbound links).
* It’s nicer, and more consistent.
Please make sure that if your site has already been indexed by Google without the www, this might cause unwanted side effects, like lost of PR. I don’t think this would happen, or in any case it would be a temporary issue (we are doing a permanent redirect, 301, so Google should transfer all rankings to the www site). But anyway, use at your own risk 
!
Another nice way of writing .htaccess is code above is that you can use it for any website, since it doesn’t include the actual domain name.
Redirecting www to non-www
If you want to do www to non-www, the code is very similar:
in your .htaccess file
RewriteEngine On
RewriteCond %{HTTP_HOST} !^mysite\.com$ [NC]
RewriteRule ^(.*)$ http://mysite.com/$1 [R=301,L]
In this case we are explicitly typing the domain name.
I’m sure it is possible to do it in a generic way same as above, but I haven’t had the time to work one out. So remember to change ‘mysite’ with your domain name!, and I’m keeping this post open for all my readers to comment on it and post generic code for www to non-www redirection.
