Post a video of your squat, this shouldn't be happening. Squats greatly strengthen glutes, so your hips should be permanently thrust forwards from glute activation.
In addition, good technique in the squat, deadlift & press will greatly strengthen abs, keeping your anterior abdomen nice and rigid in normal standing.
Good technique in squats, deadlift, bench press, rows & pull ups will greatly strengthen the mid/upper back, improve thoracic mobility and teach you to keep your chest big and out at all times.
If your posture is becoming fucked up, Im guessing there was a problem in the first place or you are using horrible technique.