(Concerning the ecchi drawing) Her spine is broken and the proportions are skewed. There is very little anatomical sense, and there isn't much attention to perspectives etc. I would have a look into studying anatomy and go back to the foundations and work up. Have a read through Loomis' Figure Drawing: http://illustrationage.files.wordpre...-its-worth.pdf it's a great resource for those starting out.
Look into anatomical principles such as gestures:
And make sure to use human references when studying the human body; whether you choose to study naked or clothed models is completely your choice. Remember that if you are wanting to stylise drawings then you need to understand what you are drawing first. You cannot make an interpretation of life when you haven't observed and understood the basics of life. Also read this other book by loomis: http://www.alexhays.com/loomis/Andre...a%20Pencil.pdf
It will help introduce you to basics of art, such as deconstructing your model and building drawings from form. Simple to detail.
EDIT: i've been too busy to properly elaborate on some details, so sorry for the simple and quick review