Main principles

Note: the principles are order by their importance (from my point of view).

• Eat frequently - every 3-4 hours. If you convert that into meals, than you'll get 5-6 daily meals. For example: breakfast, 10 o'clock meal, dinner, 16 o'clock meal, supper. Or something like this: 8:00, 10:00, 12:00, 15:00, 18:00 and 21:00. By telling meal I mean every amount of calories. Fruit, cup of milk can be quick healthy snacks and considered as a meal. If you are changing from usual 2-3 meals a day to 5-6 try to maintain (and not increase) amount of daily calories (same amount divided to more meals).
  There are many advantages for frequent eating, among them: increase of metabolism (according some sources 10% increase) thus decreasing regeneration time and easier to decrease body fat. Also, due to frequent eating you shouldn't feel hunger between meals and therefore it will be easier to control portion size. Figurative explanation of frequent meals when the goal is decreasing body fat: ancestors didn't have steady food (today you have food (as result of hunting/fishing...)), but tomorrow you might have none or less that your body needs to sustain itself), therefore body fat served as food reservoir. When you eat every 3-4 hour your body adjusted to constant food income (no drastic changes in insulin level) and as a result decreases the amount of food reservoirs (body fat).
• Drink water - drinking liquids is important. Of course water is preferable over soft drinks, coffee and etc. Drinking insufficient amount of water (more generally, liquids) will not only harm your health, but also causes your body to burn less fat. Additionally, many people confuse between hunger and lack of liquids. And thus instead of drinking are eating. How much water should you drink? Many advice drinking 3-4 liters of water daily, but I usually use a different parameter. During a day the color of urine at least 2-3 times should be white. If you don't get that and the color of urine almost always yellow, than you aren't drinking enough.
• Common sense - there are many different books, nutrition "experts"... you should analyze any principles before applying them. Additionally, find someone whom you trust and get his advice. I've met people who tried cabbage diet. It's a diet for several weeks in which in one of the days you should eat only cooked cabbage, in other day you should drink only cabbage juice and so on. Of course this diet will work. You'll lose weight, but this will be at the cost of your health. How much damage will you cause? I don't think anybody can tell you in advance. Therefore, prefer balanced diets that include all nutrients.
• Control portion size - most people put on their plate more food than their body really needs. Most people estimate amount of food by its view (using their eyes), but view can be deceptive. For instance: if you take bigger plates, than it will look like there is less food. There are different tips to control portion size, among them:
  1. Using food scales - this is the most accurate method and used by professionals. The downside is its inconvenience, all food that you eat has to be prepared in advance (weighted and divided to portions at home).
  2. Comparing portion size to some handy object. For example: "fist system" (taken from: "Body for life"). Let's say you're eating rice and a steak for dinner, then the amount of rice is as the size of your fist and the steak is has the same size as your palm.
  3. Using dishes as volume measurements. When eating at home you can use dishes which approximate "load volume" (the amount of food that can be placed) is know in advance. You need to make only one test after buying these dished. As a general rule its preferable using smaller plates than bigger ones. Everybody knows: spoon = 20 gram, teaspoon = 5 gram and standard cup = 250 gram but very few use these measurements. When according to your meal plan you're supposed to eat 150g of mashed potatoes, why not using a spoon as a measurement tool?
• Learn to read labels - in many cases there are big differences between the amount of calories we think that this food has and the actual one. There are also other issues, like: the same food category might have different energy level. I'll explain myself, for example: while choosing rice in the store I've encountered packages of several competitor firms. It was a small discovery for me that simple rice might have vast range of energy (calories) value. Calories per 100 gram varied from 150 kcal to 450 kcal. Additionally you can't use a diet in which you need to count calories without reading the labels. Even if you don't count calories (though it's highly recommended) reading label will give you a better feeling of what and how much you eat. Lastly, many experiments were done and showed that the food which is labeled as "healthy" or some other special label (and by the way this food is more expensive than other food from the same category) usually has no significant difference (in some cases it's even worse) than other food from the same category. Therefore always read labels, including small letters (for more information see: what is a calorie section).

Other principles

• There is an old saying: "Eat breakfast as if you were a king, lunch as if you were a prince and supper as if you are a poor man" (free translation). It's a good practice to decrease amount of consumed calories toward evenings. Also, it's advisable to have a break of several hours between last meal's time and bed time.
• 30-40-30 ratio: this proteins-carbohydrate-fats ratio (30% of consumed calories should come from proteins, 40% ...) is recommended by many dietitians. This ratio might vary, but keep some limits. For example: don't let any of these numbers go above 60%.
• Steady meal hours: when you becomes used to steady meal hours, all digesting related processes become more efficient (human body adopts and makes "optimizations").
• Prepare your own food: when you prepare the food, it's easier to control its ingredients. You can know exactly how many calories each meal has.
• Sauces: sometimes this is the source of many "empty" calories. Have you ever thought that gravy might have more calories than the meat it served with? Or that salad souse has more calories than the salad without the souse. Keep an eye on the small details as well.