It sounds simple, but years of habit can take a bit to correct. In fact some studies show it takes thousands of repetitions to correct bad habits. We spend 12 hours a day upright, this is key time to keep your abdominal wall from being overstretched. With your feet parallel, stack your hip bone over your ankle bones. Stack your rib cage over your pelvis, careful not to flare the ribs. Breathe normally.
Don’t stand with a swayback and open rib cage, “no rib flaring”. This exacerbates the issue. The smallest of changes – closing the ribs and stacking the hip bones over the feet – takes pressure off the linea alba (where the recti separation occurs).