Poise over Posture
Often times when I am sitting around playing guitar I think about..well, how I am sitting. Posture is a big deal when it comes to guitar. As for the hand that is pressing down on the chords, your wrist needs to be pushed out; bringing your whole hand around to meet the strings at a near perpendicular angle. This is important because it keeps your fingers from dragging on the strings, thereby muting or altogether messing up your tones. Having your wrist like this feels odd at first, but it will ensure that you are getting clean notes, and that your posture is starting to shape up.
Now that you have achieved a clean sounds when you strum the guitar, it is time to work on getting your posture right. I have seen people perform brilliantly with their shoulders back and a rigid spine, while other seem to be slumped over their guitars. Either way I say be comfortable. When you are comfortable your muscles don’t tighten up and it looks and feels natural. Stay poised and balanced. If you are going to be a great guitar player find a good chair and get comfortable. You are going to be there a while.