I think portion sizes and snacking are the biggest culprits. If you can get your portions and snacking under control, it will go a long way.
I am thin, and admittedly have never struggled with my weight. I do think that there is a genetic component to this, but I also think my habits are quite different to the habits of many others.
Today I ate:
All bran with strawberries and full fat milk, plus a banana - breakfast
Ham sandwich - lunch
Cadbury chocolate - afternoon snack
Chicken schnitzel and chips - pub dinner
Bulla chocolate coated vanilla ice cream - dessert
Not the best example of what I typically eat. It tends to be more like this:
Porridge with banana - breakfast
Fruit (oranges, mandarins, strawberries, blueberries, kiwi fruits, pears all featuring heavily at the moment), 2-3 tablespoons of plain vanilla yogurt (not Greek, just plain ol' yogurt), a Birds Eye steam fresh bag of veggies, and either a banana or a Vegemite sandwich - lunch
Dinner could be anything - spag bol, lasagne, chicken pie, meatballs, big plate of roast veggies, sausages and mash, steak etc. all feature quite often. I eat a lot at lunch, so dinner is quite small (half a piece of NY steak, one sausage etc. bulked out with lots of veggies).
I will typically have something for dessert.
Because I eat like this 80% of the time, I don't watch what I eat the other 20%.
But I also walk between 6 and 8.5km every day.