The only acceptable form of weight loss relates to losing fat, as losing muscle mass is not desirable and losing water to the point of dehydration is very harmful to the body.
The most effective methods for losing fat are changing nutrition permanently and exercising regularly with much aerobic exercise (cardiovascular activity).
Diets should not be recommended as they're only temporary and rely on an excessively reduced general calorie intake, which causes hunger and deprivation.
The idea of weight loss has spawned a multi-billion dollar industry, selling exercise equipment, supplements, diet plans, services, fashion, and gym memberships. The advertising from this industry has encouraged some people to overdo it, leading to exercise-related injuries, anorexia, and body dysmorphic disorder. Young women are particularly vulnerable.
Misconceptions
Infomercials generally promote the idea that a fast method to get a flat stomach and visible abdominal muscles is using special devices which target the core muscles of the body and burn the fat in the region. This is false: removing fat from a specific region is impossible as it's the body that decides which fat should be burned off. In general, the first place where excess fat is stored is the midsection, and it's also the last place where it comes off.
Some claim that to get a hard midsection doing many crunches is the best method. This is false: the abdominals are already present under the layer of fat, and the only thing to do to make them visible is removing the fat through good nutrition and cardiovascular exercise. It is important to say that crunches aren't useless, in fact they are very important for the abdominals, as a strong core can help prevent back pain and improves performance in many sports; they're not important when it comes to burning fat, as they can't push the heart rate high enough.
Nutrition
Nutrition a very important part of weight loss and specifically fat reduction. Good nutrition doesn't require any temporary diet, instead it needs a permanent approach by changing daily eating habits. It needs to be quite strict as any kind of foods containing empty calories (pizza, hamburgers, ice creams, alcoholic beverages and foods containing too much sugar or fat) must be avoided or at least be consumed no more than once every two weeks.
To increase or diversify metabolism, a person should go from 2-3 big meals a day to 5-6 small but balanced meals a day. The bigger portion of every meal should be comprised of vegetables and whole grains. Every meal should have lean proteic components such as beans, dairy, or meat. There should also be a portion of fruit. Fats should comprise less than 20% of the total caloric intake (only good fats like those found in flax, avocados, olives and nuts).
The less a person weighs, the less they need calories to lose additional weight. A 120 pound person needs to consume less calories than a 200 pound person, in order to lose weight. In many cases, the body can reach a limit where it starts to keep a little additional fat for survival purposes, and it can actually burn muscle tissue instead. A tested, safe method to lose the remaining excess fat in the lower midsection is the carb-cycling diet.
Exercise
Aerobic exercise is important as it allows to burn calories and has various benefits, like improving lung capacity, reducing the risk of heart diseases, and reducing the risk of diabetes. Proper aerobic exercise executed with the target of burning fat needs to elevate the heart rate of a person to more than 70% of their maximum heart rate (which varies depending on factors such as age and health condition).
In order to be effective, aerobic exercise must be performed continuously for at least 30 minutes a day, preferably 45-60 minutes.
Examples of good forms of exercise which help burn fats are: walking at a fast pace, running, cycling, swimming, soccer, basketball, football. Cycling, and swimming in particular, have an advantage over sports where running is required in that there's much less stress on the leg joints and risks of injuries are reduced greatly.
Another good form of exercise is High Intensity Interval Training, meaning that for example for every 1 minute of very fast running there's 1 minute of fast walking. Some research has shown that it burns off more calories in a shorter span of time. Good activities where HIIT can be incorporated are cycling and swimming, as they have much less impact on the knee.
Weight lifting is important as well because it helps burn off additional calories when combined with aerobic exercise. A larger muscle mass requires additional expenditure of calories for the body to maintain it.
Other information
Losing fat and getting a flat stomach (text)
Losing fat and getting a flat stomach (video)
What is good aerobic exercise? (video)
Removing fat from certain places?
Carb cycling (video)