I seem to be hacking my .htaccess file a lot these days and usually get stumped over something that turns out to be stupidly simple. This comprehensive (and comprehendable, to me anyway) tutorial looks to be rather essential in this regard.

A friend called today saying her somewhat sexy photos were being used on other sites causing her bandwidth to go through the roof (5 gig so far this month!). Within 10 minutes I'd implemented this system and, while a little crude, it works. Nice one. (Needs a host running php and Apache but, to be honest, if yours doesn't do that you should consider moving.)

Modwest support page for Domain Aliases, specifically useful for redirectiing to a subdomain or directory.