"I wore a double corset day and night for three months," Alba told Net-A-Porter. "It was sweaty, but worth it."
Despite the efficiency of her method, the 31-year-old actress claims "It was brutal" adding that "it's not for everyone."
Alba is the mother of two girls, 4-year-old Honour and 20-month-old Haven and will return to the silver screen as Nancy Callahan in the upcoming "Sin City" sequel, "Sin City: A Dame to Kill For," out October 4.
For Alba's entire interview visit Net-A-Porter magazine.
Susan Sarandon 'very excited' about daughter's pregnancy
Interpol investigating stolen passports on missing flight