Repetition. We talk about healthy food choices all the time. Limiting sugar and processed crap. Avoiding fast food. We try to buy local bread with just a few ingredients, local produce and meat. My 11 year-old shot his first deer this year, a huge buck, and so our freezer is full of lean healthy venison. Well, not so full now.
Our kids are 7 and 11. While they love their deserts, cheeseburgers, wings, stuff like that on occasion, they also readily eat and enjoy sweet potatoes, broccoli, asparagus, squash, apples, fish, almonds, venison, and lots of other healthy whole foods.
I think it is important, as they say, is to shop on outside of row of the grocery and avoid all the processed crap on the inside. McDonald's should be like fair food. Eat it once or twice per year.
As a parent, you have to be committed to it because it takes persistence.