A wide variety of leg-(or-knee)-raising exercises (commonly found in many ab-programs); & Chinese 5-Animal Play "Ape" posture breathing-exercise (legs fairly straight/stiff & spread-out quite-wide, bent-forward at hips, with hands fisted at hips, thumbs-outboard, spine straight: breath & try to feel muscular tension in lower abs with each exhale.
Also pay attention to your nutrition carefully, as this may be a fat-storage area for you.