Maintaining good posture while playing guitar is vital. Not only does it help your guitar playing be more effective, but it also helps keep you healthy.
Here are a few tips you should keep in mind...
1. When sitting down, be sure not to lean your left arm/elbow on your thigh while you play. Doing this not only makes your arm less mobile, but it also has the potential to hurt your arm. Putting unecessary amounts of pressure on your arm for long periods of time can cause some damage. The next tip will help...
2. Keep the guitar neck parallel with the ground, or even better, angle it upwards at all times. Never have it angled downwards. Doing so will only encourage you to lean your elbow on your thigh.
3. Keep the pinky on your picking hand stationed on the body of your guitar as much as possible. This is mainly for when you are picking individual strings. Obviously when you're using techniques like finger-tapping, strumming, or things like that, this won't apply. But when you pick strings individually, keep that pinky down as a reference point for your picking movements. Doing this will help your picking be a lot more accurate.
4. Make sure you keep your picking-hand wrist as straight as possible at all times. Make sure it's relaxed, but keep it straight. Some people tend to curve their wrist a lot when they're doing standard strumming. Do your best not to do that.
5. Keep the elbow of your picking arm anchored on the top corner area of the guitar's body. Let it rest there gently. There's no need for the elbow to be bouncing up and down while you're strumming. Keep it gently anchored.