What is the correct standing posture for yoga/meditation?

I am trying to get my standing posture correct for yogs/meditation. How do you stand to get the proud holding of lifting your adbomen over your pelvis? Do I tuck my butt in and pell my navel to my spine? Or, is it more analogous to how a pregnant woman walks with her belly first or to how a model stands? Also , when sitting cross-legged, do I sit up straight and relax or sit up straight and rigid?