Er, and lots of dieting and exercising! So not that much of a secret, really!
The superstar singer said she shed pounds in super-quick time by feeding baby Blue Ivy the natural way since she was born in January.
"I lost most of my weight from breastfeeding and I encourage women to do it – it's just so good for the baby and good for yourself," the 30 year-old told People magazine.
But she also admitted to dieting in her quest to get her body shape back.
"I counted calories. I worked out maybe three to four times a week," she said. "I did a lot walking in the beginning and now I'm running.
"But I had to work my way up. I couldn't just go right from being pregnant to running."
Beyoncé also told how she tried to look as glamorous as possible when she went into hospital to give birth.
"I did have a fresh eyebrow wax. I got my nails done, I got my feet done, had my hair done and I had my little lip gloss," she said.
"I didn't feel that I looked beautiful during birth but who does?
"After being pumped with all those fluids and gaining so much weight. I barely recognised myself. But after many hours of labour I could care less about anything but my child. I didn't care how I looked."
Did you do a Beyonce 'beauty prep' for your birth?
Did you lose weight easily thanks to breastfeeding?