Instead of trying to finish your plate because it tastes good or you do not want to waste things, stop eating when you START feeling full. It has helped me a lot, and changed my eating habits greatly. I now get smaller portions. This is especially important to think about when eating out. Take home the extras for another meal, don't try to finish the plate or prevent some from 'going to waste.'
A related tip: avoid 'deals' on food. Getting more for less money is not necessarily good. Buffets are a big one, avoid those! Pay more for less quantity, more quality!