Seconding the suggestions to occasionally flip your drawing upside down and to use guidelines. Look into construction method drawing. Obviously having a mirror handy can be helpful, but I don't find it as useful as the digital canvas flip personally.
About posture... Pay attention to whether you tilt your head when drawing, as I do. It's an unconscious habit, but it definitely affects my work. I've heard some people say it won't actually cause skewed art, but I wholeheartedly disagree. Whenever I realize I'm doing it, and correct it, my work always turned out more balanced.