There's a catch, though. The portions have to be of food that is healthy and low in calories. What does this mean, practically? A study found that our hunger is not based mostly on how many calories we eat, but rather on the volume of the food. If you eat food for which you can increase the serving size significantly, without increasing caloric intake, you'll be less hungry.
This means that next time you're choosing between a small or large salad, go for the large. The portion size will make you less hungry. Watch out for the dressings though, they'll get you.
Another thing to keep in mind: cutting up your food can have make you feel more satisfied. A study gave people a bagel; it would either be whole or chopped up. The people who ate the bagel that was cut in pieces said they were more satisfied than the ones who ate the whole one.