With an ideal posture, the body is in the best position for the activity being undertaken. Stresses are distributed in the safest/most efficient manner, and the body is as strong as it can be.
There are many disciplines that represent ideal postures, demonstrations of the body's capabilities when it is functioning at optimal. For example:
A 'functional posture' is what the brain/body uses day-to-day when an ideal posture cannot be achieved. (When the five main muscles of movement are not adequately used, and physical restrictions are present on the body).
A functional posture at its most basic:
Subconscious adjustments are made throughout the body - twists, kinks, tilts and compressions - as the brain sees fit, using the 'wrong muscles' in an attempt to compensate for misusage in the main muscles, but the body is imbalanced and imbalance leads to further imbalance.
When faced with a task, the brain/body prepares by activating muscles into an anticipatory posture. "Bracing yourself".
An anticipatory posture should be the ideal posture for the activity (using the main muscles of movement to their full potential) but if that is not achievable, the body braces into a functional posture with the use of other muscles that attempt to mimic the action of the main muscles.
Becoming aware of anticipatory postures and the activation of the wrong muscles is an important step in correcting the dysfunction. Breathing with your Base-Line and focusing on the location and activity state of the main muscles of movement will facilitate the correction of bad postural habits that have developed.
It takes time and effort to improve your posture, things only you can provide. Think of how you are put together and what level of body awareness you have.