'People who don't have a weight problem are the logical source' ... are they? I would have thought they are naturally inclined not to put on weight so they won't have to control weight?
I did a hub on this a short while ago: http://hubpages.com/health/Avoiding-Christmas-Holi...
Anyway, to answer your question, my personal experience to keeping my weight down comes down to what's in my shopping basket and portion control. This is something that should be done ALL the time and I also notice it with slim people, they tend not to eat big portions. There will always be times when you get to 'over-indulge' but hopefully by then, you would have been used to eating the healthier and smaller portions that you won't beat yourself up afterwards.
Shopping basket::::: take the healthier options (e.g. extra virgin olive oil instead of vegetable oil, baked chips instead of fried, etc) and the smaller pack sizes. You are basically stopping yourself have the foods readily available to reach whenever you want it. If you do have it, it will be smaller.
Smaller Portions::::: whether healthy or not, you are what you eat when it comes to portions. Assign yourself to eating with smaller sized plates that restricts you from piling on the food.
Alcohol::::I basically have none, if I do, it is less than 10 times a year especially if it's a cocktail mix I've never tried then just want to see what it is like. I was raised by my parents with Alcohol but just didn't like how it made me feel and stopped drinking it.
Exercise::::: this is not really dieting but you have to get your metabolism going with the little things like walking instead of driving, taking stairs instead of lifts, etc. I like going to the gym as it clears my head but I realise it can be tedious so I can't just say to everyone "you must go to the gym!", if you can, great, if you can't, find the little ways to get your system pumping.
I don't think I've ever gone out of my way to diet or eat lots of fruits and I don't like vegetables but I stay a small size always by doing these things. I am no freak about my food being natural or not, it's hard to determine what is completely natural if you are buying it from someone else and not making it yourself.