My kids are in their late teens so we can see the impact of our approach to healthy eating.  This is what we did:
1. Fresh food is best.  We prepared meals from scratch whenever possible.  Processed foods were an exception.  
2. Snack foods included fresh fruit, crackers and nuts.  Unless home-made, cookies were rarely in our house.
3. Sweetened breakfast cereals are not food, they are highly processed snacks.   We'd have sweetened cereals on vacations or holidays.  
4. Potato Chips, junk food and sugary drinks such as pop and iced tea were rare exceptions in our house.   They'd have as much as they wanted on birthdays, holidays or special occasions.  We let our kids know that these foods were not healthy.  We'd add lemon or lime or Pom to soda water.   We'd make popcorn and add any number of combinations including butter, maple sugar, chocolate...
5.  Eat what you need. No one was ever forced to clear their plate, even if only a mouthful remained.  

It seems to have worked...