The #1 habit of healthy people is Intentional Eating! They know exactly what type of food they put in their body. Healthy people often tell themselves "no" and skip the processed junk food. They make sacrifices to keep their bodies healthy.
Healthy people read nutritional labels and scan for artificial sweeteners, food additives, and preservatives, because they prefer avoiding chemical crud. Healthy people aren't always 100% perfect with their nutrition, but they do their best to avoid food that is unhealthy.
If you want to start developing this healthy habit, pay close attention to the foods that you eat. Keep a food log or track on a website like My Fitness Pal or Fitday. Start documenting the foods that you eat and how often you eat them. Remember to focus on adding in healthy fruits and vegetables rather than just focusing on eliminating foods. That's Intentional Eating!