I've heard the Japanese say they eat until they are 4/5ths full.
Many westerners on the hand scoff food until they couldn't squeeze another bit in.
I heard a good thing on the radio the other week from I dietician people just over eat always "snacking" - she said if you want too eat EVERYTHING you eat goes on a plate you get a placemat sit down at the table and eat it ----- she reckons this cuts out a lot of the excess when people have to actually think about it.
I fast 24 hours two or three times a year just to wash out the insides.