A few cues may help you. With your spine level, take a deep breath in. As you let the air out, pull your belly button up and in. Think about touching the back of your spine with your belly. Think, "belly in, breath out."
It's a little like patting your head and rubbing your belly at the same time. It does take some time and practice to do it naturally without the verbal reminders.
As you bring your belly button up and in towards the spine you are actually contracting the abdominal muscles. These particular muscles help stabilize the spine. Coordinating the breathing helps keep the respiratory muscles from working against the abdominal muscles.