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Sienna Miller returned to the spotlight for the first time since the July birth of daughter Marlowe Sturridge (with actor Tom Sturridge) when she stepped out to promote The Girl at the Hamptons International Film Festival this weekend. And her return was a stylish one: She chose an Alessandra Rich cape-style dress for the film’s premiere and followed up with Sea NY printed trousers ($375 at matchesfashion.com) for the festival’s brunch. So how does it feel to be back? “To be honest, it’s kind of nice to put on a dress,” she told InStyle.com. “I’ve been in baggy jeans and baggy T-shirts and breastfeeding—it’s hard to have any semblance of glamour when you’re in the middle of new motherhood. It’s really lovely to be able to put on a dress and feel like a woman as well as a mother.” That said, her red carpet approach has changed somewhat. “I’m just so focused on my daughter that I don’t care as much—which is great,” she added. “She’s more important.” And then there’s her secret weapon: “Spanx!” she declared. “I’ve had to get a lot of Spanx. That’s rule number one.”

Plus, see Sienna’s best looks ever.

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— Violet Gaynor