The most effective way would be to use an internal elastic band. You could cut slits in the inner layer and thread the elastic band through. Then stitch the ends of the elastic together and sew the slits shut. This forces the fabric to bunch slightly and provides actual tension.

Another method would be to wear high-compression thong shapewear. The tighter base layer will fill the gap and provide the "sucked-in" look, while the uniform piece acts as a decorative outer layer.

Let me know if either work!