how to automatically add www prefix?

[dinodonk]

how to redirect all mydomain.com to www.mydomain.com

i have tried :
Options +FollowSymlinks
RewriteEngine On
RewriteCond %{HTTP_HOST} ^mydomain\.com [NC]
RewriteRule ^(/.*) [mydomain.com...] [L,R=301]

it only works for domain.com/something , not work for domain.com/forum/something

TIA

[jdMorgan]

For use in .htaccess:

Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^example\.com [NC]
RewriteRule (.*) http://www.example.com/$1 [R=301,L]

Jim

[dinodonk]

Thanks jim,
it works for example.com/forum ( no trailing slash )
but that doesn't work too for
example.com/forum/showthread.php
example.com/forum/

Regards