A lot of people recommend that you drink a glass of water before you eat to fill your stomach so you don't eat as much. I've never understood this advice. Water, or any beverage really, just doesn't fill me up the same way food does.
Eat fresh fruit or raw vegetables first
Fruit and vegetables are bulky foods that have few calories. They are incredibly filling.
Try to stick to raw vegetables. Cooked vegetables aren't nearly as filling. And depending on how they are prepared, they may have a lot more calories than the same vegetables in their raw form. If you must cook the vegetables then steam them.
Drink a protein shake first
My daily protein goal is 195 grams. My normal diet usually doesn't come close to this so I supplement my diet with Whey protein powder. After I drink an 8 once protein shake I'm stuffed. If my meal include a protein shake I eat it first.