When you eat a plate of food. Always leave one spoonful uneaten.

Not only will you drop a few percent off your daily calories, but we are conditioned as children that we cannot leave the table until we eat everything on the plate, which apparently damages our ability to recognise being full. Leaving a little bit each time does seem to help break down that conditioning over time. You don't need to finish it all.

Other tips to feel full sooner, take longer to eat something and precede meals with a glass of water.