mod_rewrite (htaccess) and subdomains

[eluser]PorkOne[/eluser]
Hey
I know that this topic is back again on this forum but I didn't found working solution.
How to get working something like this?

http://user_name.my_domain.com

I would like to this url read data from:

http://my_domain.com/users/show/user_name

How to make it working without "index.php" segment?
Can anybody post full working sample?

Thanks