I picked up this charming little book from the Victoria & Albert museum the other day. Divided in letters like a dictionary, it's the closest I've found to a good reference guide to style. It gives you a clear perspective on things and it's very good to unclutter your mind.
Some favourite quotes:
Nonsense: I could write a book about nonsense in fashion! (...) Good fashion is always a natural evolution and based on common sense. I dislike trick fashions - designed only for publicity's sense. They may be eye-catching but they are never elegant.
Individuality: Until the time we are converted into robots - and I hope this time will never come - individuality will always be one of the conditions of real elegance. (...) Try to understand well what is your personality and never forget that individuality does not mean eccentricity.
Fit: A good dress is, first of all, a well-fitted dress. (...) It is the fitting that helps to emphasize the loveliness of yourself and to hide the little faults you might have.
Elegance: (...) I will only say now that elegance must be the right combination of distinction, naturalness, care, and simplicity. Outside this, believe me, there is no elegance. Just pretension.
Colours: Colours are wonderful and glamorising - but they must be used with care. Even the prettiest colour, if you wear it every day, will lose its effect. Colour needs change. We wouldn't appreciate blue skies if they were always blue, it is the clouds, the ever-changing scene, that makes the sky so beautiful.