Upper Crossed Syndrome is a great place to start and my be the reason. If the person is already fit then I would look at the current program and see if the horizontal and vertical push to pull ratios are even. If not then I would start from scratch by activating the rhomboids, mid to lower traps, lats and rotator cuff muscles to strengthen the back side. I also make sure to shoe my client how to set thier shoulders and make sure that they hole that position in certain movements to train the correct musculature. Also Mackenzie stretches, chest stretches and hip flexor/abdominal stretches my help with the posture.