Waist Trainers

So I used to play soccer and was a size 3 back in the good old days. Now that I'm out of high school (just turned 21) I am a size 7 going on 9... I'm still not very flabby but for someone who is not taking this very well I'm planning on making some changes. One of them just so happens to be using a waist trainer. Has anyone tried using these? What are your thoughts an opinions? I was told they can correct your posture which is my main reason for wanting to try them.