There are all kinds of treatments for the face, and the most invasive is a Deep chemical peel. Yes, the down time can be hard and the effects during can be shocking, but one cannot deny the results. Perfection.
Microdermabrasion and laser resurfacing isn't as bad, and I suspect Mary may have had microdermabrasion to fix her sun damaged skin. In earlier photos, specifically during the engagement press conference, she had noticable Hyperpigmentation. A couple of treatments is all it takes to rid of HP, and as we have all seen on the day of her wedding her skin was amazing. Completely opposite of what it looked like back in October 2003. Now, I have heard others claim that her skin magically changed, and thus the hyperpigmentation, sun spots, and so forth, disappeared thanks to Denmark's weather. Nice theory, but scientifically and logically impossible. One's skin doesn't change overnight like that without the help of Dr. Skin Rejuvenation.
As I said, before, Mary obviously takes good care of her skin, through I imagine, a time consuming routine. Lots of sunscreen, eye cream, facials, day cream, night cream, healthy eating, water, etc., etc. Anyone can achieve that "amazing" skin look, but only if one puts in the effort, then one will succeed.