Shiny, satin-finish hair has become Kate Middleton's signature beauty look. Whether she's attending a royal engagement or stepping out for a more casual appearance, her hair always looks smooth, healthy and effortlessly polished. Surprisingly, that's not from over-the-top treatments or impossible routines. Her secret is a luxe shampoo you can actually buy on Amazon. According to Grazia UK, Middleton uses the Kérastase Nutritive Bain Satin Shampoo - a pick known for delivering softness and shine without weighing hair down.