I've never really had to diet (lucky, I suppose), but I have seen many of my friends dieting, with varying success. One thing that
does not work is simply trying to eat less, of course. Or even trying to exercise more without adjusting your diet at all. It will just make you hungrier, and you will relapse.
The key thing is to make the restrictions on your diet reasonable. It's stupid to swear off all fat-making foods or drinks, because that's way too unpleasant and unreasonable. Nobody can keep that up. Without becoming anorexic or something.
So what you really have to do is try to get to the root of the problem without overcomplicating things. One good way to do this is by simply counting calories. Of course, you have to watch where those calories are coming from, but as long as you don't eat/drink empty calories, cutting down on your calorie intake a bit will not leave you hungry. I strongly recommend Livestrong's "My Plate;" it makes tracking calories very easy and also gives you other useful information about your intakes of different stuffs.
The most important thing, though, is not to overdo it.
>>1 That's too much weight in a month to be healthy, but
>>3, it's nice to lose more than that, while it's still not unhealthy. It's probably better to shoot for a couple pounds a week.
The calories-tracking method has worked very successfully for multiple people I know, and have kept those people at a healthy weight for a while since; however, other methods—like the Atkins diet, and even just anorexia or bulimia—have had very little immediate effect and none whatsoever in the long run.