Over the past 50 years, portion sizes have doubled. It started in our restaurants and worked it's way into our homes. Our view of what is a "normal" or "average" portion size is very distorted. Due to this, and other varying factors, our obesity rate in America has more than doubled since the 1970's.
Above is a diagram of what your dinner plate should look like, when it comes to portions. One of our biggest problems is that we are usually putting way too many starches on our plate, and not nearly enough veggies. Another trick I like to use at home, use a salad plate instead of a dinner plate. The smaller size will automatically give you smaller portions.
Here is a guide for portions, when it comes to snacks. This can help a lot, because like our meals, our snacks are also way too large.
These portion sizes will probably look very small to you at first, but you will get used to them. So next time you are preparing your meal, keep the diagram of the dinner plate in mind. These little changes over time, lead to big results. When dining out, ask for a take home container immediately and put half of your food in it. You will find that at the end of the meal, you are comfortably full, and you really didn't need all of that food.