.htaccess - how to force "www." in a generic way?

I would use this rule:

RewriteEngine On
RewriteCond %{HTTP_HOST} !=""
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTPS}s ^on(s)|
RewriteRule ^ http%1://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

The first condition checks whether the Host value is not empty (in case of HTTP/1.0); the second checks whether the the Host value does not begin with www.; the third checks for HTTPS (%{HTTPS} is either on or off, so %{HTTPS}s is either ons or offs and in case of ons the s is matched). The substitution part of RewriteRule then just merges the information parts to a full URL.

This will do it:

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