@hawkeye10,
The problem is really calorie intake. If you take in more calories than your body can burn up, regardless of the source of the calories, whether from fat, or carbs, or protein, you will start to gain weight.
People should know what their daily calorie intake should be for a person of their sex, age, and activity level. Then, if they keep track of their calorie intake, and try not to go over a certain limit each day, they won't get fat, regardless of what they they eating.
Trying to eat a healthy diet, however, is a different matter than just trying to control weight gain.
I not only try to keep some track of the calories I'm eating, I also watch my intake of fat, sugar and salt. Foods high in fat are also higher in calories. If I can eat only about 1200-1500 calories a day, to maintain my weight, I'm not going to blow half of it by eating a piece of cheesecake. I think of spending my daily calories the way I think of spending money from my checking account--I don't spend more than I have (go over my calorie limit), and I want value for my buck (I want as good nutrition as possible from those alloted calories, and I don't want to feel hungry). So I try to make smart choices. I buy whole grains, eat as many veggies and fruit as I can, eat more chicken and fish than meat, avoid fried foods, have reduced fat dairy products, etc. This has been a way of life for me for years. It's a lot easier than most of the reducing diets I went on earlier in my life. If I can avoid gaining weight, it's easier than taking it off later.
Mainly I think about whatever I put in my mouth and avoid mindless eating, and I try to make everything I eat something that tastes really good to me. If I really want a cookie rather than an orange, I'll have the cookie, as long as it's about the same calories as the orange. That automatically limits the size and type of the cookie.
If I want potato chips, I'll buy the baked potato chips, rather than the fried, and eat a reasonable amount, and take the salt and calorie count into consideration. If I want some ice cream, I have the light ice cream. Because I'm keeping an eye on calories, portion control really takes care of itself, as long as I maintain some semblance of self discipline. I read all food labels before I buy anything.
I think the real problem is eating out. I wish all restaurants had calorie counts on the menu. I can estimate it in my mind, but that's just a guess. When I do eat out, I try to avoid the high fat items (because they are higher in calories), and I'll try to consume fewer calories the next day.
The key is moderation, and some self control, as well as thinking about what you are eating.