Portion Control

Since calculating body gain and loss is basically a mathematical equation, portion control is one of the most important factors in maintaining a healthy weight. If you want to lose weight, your caloric intake must be less than your calories burnt. If you're simply trying to maintain your weight, you just need the calories you burn to equal the calories you consume.
When you're cooking for yourself, or at a place that has large portions, even "all-you-can-eat" buffets, it is important to remember the size of your stomach, and to not stretch it out. One of the ways to do this, if you aren't going to avoid these types of places all together, is to look at the appetizers or small entree parts of the menus. Restaurants such as Cheesecake Factory have the "small plates and snacks" section, and seeing as their portions are huge to begin with, the appetizers and small bites can be perfect for a meal that won't stretch your stomach and will be affordable.
Another portion control trick is for when you are eating at home. By buying children sized plates and bowls. Since they are smaller, they give you the allusion that you are eating the full bowl of pasta, or a full plate of your chicken, but in reality you have cut your portion size down. It is a more casual alternative to measuring your portions with a measuring cup or weighing it.