I had braces for two years, and they were taken off when I was 16. I was then given a retainer and told to wear it a few times a week, forever.
I still have it now, though I paid £200 for a new one a few years ago. I have worn it as instructed. But my teeth have still moved :(
I asked my dentist about this, they said it's inevitable - our bones move and grow forever, especially in late teens and early 20s. Plus wisdom teeth makes teeth moveto accomodate them.
I know many people whose teeth are now as bad as they were pre-braces, especially those who didn;t bother with the retainer. It seems braces are such a waste of time and money on teenagers.