Everything is true, but there's a simple formula. Spend more than you take in. If you increase your activity level and reduce your calories, you will lose weight. The opposite is true as well.

Mark, the yo-yo thing is common enough. The largest contributing factor is lack of exercise. When you only diet, you only empty your fat cells. They will then live on, screaming to be refilled. However, when you combine aerobic exercise with diet, you not only empty the fat cells but shrink them as well.

Some hints:
Exercise early in the morning. You will boost your metabolism level for the entire day. There's also something about glycogen levels (which I don't understand) that maximizes an early-morning workout.

Don't only skip the second helpings, but eat only 2/3 of what's on your plate. Portion control is key. An American "portion" is probably twice that of anywhere else.

Don't eat after 8:00 pm.

Drink tons of water. This will flush out your system and it's even good for your skin.

Eliminate junk food. They are mostly empty calories of fat, sugar and salt that will do your body no good whatsoever.

My daughter was on medication that increased her appetite tremendously. She went from a size 10 to a size 16 in three months. Finally, they switched her medication and she lost about 30 lbs. from that alone, but she continued and in about six months went down to a size 6. She's kept the majority of it off for almost two years now, and she does all of the above.