What might help you is to exagurate the obliques. Right know many of the drawings seem like you just drew down a straight line from the ribcage to the iliac crest. In reality the bottom part of the obliques will bulge outward. You did indicate something like this in some drawings but even there you could have pushed that effect more. Also try to play with the compressing and stretching effect that proko talked about. Try to exagurate the compressed part even more but make the stretchted part thinner.