This may seem like a no-brain'er, but so many people have been "programmed" to clean their plate. Pay close attention to your body when you are eating. When it signals you are full - STOP!!!
Continuing to eat does not help the "starving children in the world;" it only makes you fatter.
Note: If you are prone to pick at the remains throw them away immediately, cover them with a napkin, or move them out of reach. If there is enough left over, stick it in the fridge for later.