RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]


e2 .htaccess
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
RewriteRule \.(css|jpe?g|gif|png)$ - [L]
RewriteCond %{REQUEST_URI} ^/[^\.]+[^/]$
RewriteRule ^(.*)$ http://%{HTTP_HOST}/$1/ [R=301,L]