If the scratch is only light then some cutting polish should suffice, Tcut or the G range work well for me.
If the scratch is deeper then you really need to use a touch up pen, some wet & dry paper and finish with a cutting polish and then apply some wax.
Have a read of this article on stone chips, the process is similar for scratches when you need to apply paint
http://www.torquecars.com/articles/stone-chip-removal.php