The clean plate club is one of the worse invention there is. Eat slowly until you are full, chew properly, stop before you get "stuffed". If you are eating quality food, this will prevent you from gaining weight. Cleaning everything on your plate is the worse measure for what your body actually needs. Listen to your body!!
If you need to develop the habit, try eating with a blind fold... you may be surprise how much food you actually leave out. Of course try not to stab yourself in the hand in the process.
You could also start with half portion, wait a bit when you are done and refill if you are truly hungry.