Correct guitar angle.
One of the most common mistakes that I see from guitar students is; holding the guitar on the wrong angle. Never hold the guitar straight across your body, or worse still; angling down towards the floor.
The correct playing position for your guitar is 45 degrees, with the head of the guitar angled upwards as pictured. This is the correct guitar position for seated or standing playing. We spend a lot of time working on this in our lessons and particularly in our guitar ensemble - The CS Music Guitar Guild.
Angling the guitar with the head straight or pointing downwards causes technique problems for both hands. Left hand thumbs tend to rise up too high behind the neck. Right hand elbows slide down the bout of the guitar and make the picking hand sit too far down the strings.
I thoroughly recommend practicing regularly in front of a mirror. It is very difficult to assess one's own technique (particularly the right hand) without some mirror practice sessions. Our guitar teachers in Caroline Springs always recommend mirror practice, this you can view yourself from the same vantage point as the teacher. And as we always say: "Remember that you are your own best teacher!"
All good technique should begin with holding the guitar correctly and good posture. Take extra care at the beginning of your technical development. Set yourself up well from the start and save yourself from problems later on. Good technique is the great facilitator of musical advancement. Don't let something as simple as poor guitar positioning hold you back.