Thanks for your question Laure. This is a very important topic
First it is important to think of posture as more than simply a position we stand or sit it but really a position that we move through. An interesting way of looking at it was proposed by Moshe Feldenkrais who said that we should really refered to posture as "acture" - posture in action.
The first thing we need to do is stand up. The average north american spends the vast majority of their time in a seated position and executives are even worse.
commute 30 min each way
executive work day 10 hours
lunch 1 hour
dinner 30 min
TV 4+ hours
Sleep 7 hours
This only leaves 30 min for standing!
In terms of exercises, spend half the amount of time doing movements that feed the problem such as chest exercises, biceps and supine crunches. emphasize movements that fight gravity as opposed to feed it. TRX examples of good movements to choose to help strengthen weaker postural muscles are as follows:
suspended lunge with lift patterns
single leg squat
balance lunge
shoulder series
postural squat
Give these a try and be conscious of your "acture" as you are sitting, standing and moving and you should see a difference
Good luck