Arched and slumped over at the same time? I can tell you from my years of dance training that the way to develop good posture is through "core" strength building and by consciously adjusting your body to correct alignment. You can lie down on the floor and press your lower back into the floor. When standing you can also use a wall to stand up against to do the same thing. Look in the mirror - relax your shoulders, even press them downward, tuck your pelvis in and stand upright with your upper torso in straight alignment with the length of your body....not sure if that makes sense...been a while since I explained this!