Most experts say you can not isolate your lower abs. Some exercises, like leg raises, seem to be felt more in the lower abs, but may actually work the hip flexor muscles more than the abs. A good all round exercise program of compound exercises (squats, deadlifts, bench press, bent row/pull ups, military press) will help condition your core, with some specific exercises for your abs/hip flexors (leg raises/hanging leg raises, situps/crunches, bicycles, etc), some HIIT and a proper diet will minimize body fat and bring out your abs.