Discipline, dedication, will-power, and more importantly the right reason for doing it and the tools for success!
The word diet actually means "the foods consumed". So everyone has a diet...it is what you eat. There are many types of diet plans for people to lose weight, gain weight, eat healthier, build muscle, stabilize blood sugar, lower cholesterol, and lower blood pressure. Not all diet plans will work for everyone.
For me personally, I am working very hard at modifying my diet to foods that will allow me to have stabilized blood sugar levels. I am eating only one meal a day that includes foods that have over 4 grams of carbs for the entire meal.
The purpose for this is that when eating, if the foods are low in carbs your body will not have a surge of insulin. By maintaining low levels of insulin for all but one meal, your body will not experience the ups and downs of your blood sugar.
These changes are what cause people to feel hunger. Eliminating the "feeling" of hunger will help keep you on track for consuming less food and not eating when you have had enough.
In some research I have done, another great idea is keeping food choices simple. This helps when in the beginning stages of a new diet plan. If you have figured out a few staples and a few meals that all equal what your body needs over the course of a day you have a better chance of being prepared for your meals.
More often than not, I know what I should eat, or how many calories I have left to consume for the day, but by the last meal of the day I am in a rush, tired, or just plain lazy when it comes to making sure I can eat right.
One great idea is to grocery shop, prepare, cook, and portion all your food for a week at a time. This helps you save on time at the grocery store, allows you to do all of your preparation once and cook all at once. Below is a picture example of 5 meal/snacks per day for a total of seven days.
Buying your ingredients for a meal seven days out might not work for all types of food. If you are eating something that spoils sooner than that, you might only prepare half a week. Since I have a husband, I often just do half a week at a time because that still equals out enough food for a whole week for one person.