There's a pattern that scientists have noticed in overweight people--gulping down food as fast as possible. That causes you to eat a lot more than common sense (or your body's physical cues) would say you need.
A really useful tip is to slow your eating down. Here's how:
Take a very small bite.
Put the fork or spoon down after each bite.
Chew extremely slowly.
Chew at least 20 times per bite before swallowing.
At first, this method is kind of excruciating if you're a fast, big eater. But it's worth a try. Though it may feel strange at first, you'll probably find that all that slow chewing actually makes you notice and enjoy the flavor and texture of your food more.