1) You must get sufficient sleep everyday. Staying awake at night should be avoided. Even if you feel fresh and energetic remember your body needs to rest. Drinking coffee or drinks containing high quantities of caffeine, only to help you stay awake at night should not be encouraged. Caffeine affects your teeth, bones, etc., as it reduces the calcium content of your body, particularly of your teeth.
2)You need to maintain a good, healthy and balanced diet. Eating a lot of junk food should definitely be avoided. Limit your carbohydrate intake but at the same time, don't get over obsessed with dieting and strictly IGNORE certain fad diets like - lemon juice diet, zero-carbs diet, etc. Along with vitamins, minerals and proteins; Carbohydrates and fats are also required by our body, in specific proportions. Your diet should be set according to your age, height and the body structure you wish to attain. However don't be too harsh on yourself. Starvation is not a method of losing weight, because it's very harmful to your digestive system and may cause acidity, amoebiosis, etc.
3)Physical Exercise is a must! You need to work out for at least 30 mins minutes, thrice a week. Going to gym for strenuous exercise is not necessary (unless your an athlete or a sportsman). Jogging, running, skipping, stretching, etc., can also be very effective. YOGA is another means of keeping your body fit as well as relaxed.
4)Stay calm no matter what! Stress and depression only hamper your physical and mental state of well being. Although mood swings are frequent and quite common, you must learn to love and accept yourself for who you are.
Happiness is a state of mind, keep smiling for 5 mins and then it comes naturally .!!
Laughter is considered as the best medicine because a happy person is a healthy person and vice versa. :)