I'm going to slightly disagree with the distinction offered by Sara. "You are looking ravishing" means that right now, specifically, you look ravishing. As Sara said, it does imply that this is a change from your normal appearance. "You look ravishing," while it can mean right now, specifically, can also mean that you "in general," "usually," or "always" look ravishing, without regard to how you look at this very moment (but probably now, also!). For example, On the telephone: You: I don't have a boyfriend because I look so ugly! Me: Nonsense! You look ravishing! (even though I cannot see you at this moment, I know that, in general, you look ravishing. I could not say "You are looking ravishing" here because I cannot see you at the moment. I COULD say "You are looking ravishing lately," and that would definitely mean that you usually do not look ravishing, but have been in recent times, and probably do right now.)

Both sentences mean the same thing.

They both really do mean the same thing. The answer that Mike has given is also true but it is a very small and very subtle difference. "You are looking ravishing" probably implies that the woman is wearing something new or changed her appearance recently. But you could also say "You look ravishing" in the same situation and it would sound fine and normal. It is also important to know that "You are looking ravishing" sounds OK but it is an exception to the normal rule. When "look" has this meaning (appear, seem) it is a "state verb" and we normally don't put state verbs in continuous tenses. Other examples of state verbs are be, own, like, hate, and (sometimes) have.

"You are looking ravishing" is in the present continuous tense, while "you look ravishing" is the present simple. "You look ravishing" means you look ravishing right now. It could be said, for example, if you were on a date with someone, or at a party with a friend who'd dressed up. "You are looking ravishing" means that recently, you have been looking ravishing all the time/a lot. It would be most appropriate if, for example, you had a friend who has changed his/her appearance-related habits in some way (maybe they started or stopped wearing make up, started exercising, recovered from an illness) and therefore looks, in general, better than they used to.