Eat less. Exercise more.
Easy advise to give I know. More difficult to follow. When I finally had some success losing weight, it was because of this site.
I had decided that there was no lucid way of discerning good dieting advice from bad. There was just too much info out there, and even my doctor seemed to be in love with diets that had shady sounding names. So all things being equal, I decided that math wouldn't fail me, and I set about looking for an accurate way to quantify my caloric intake, so that I could enforce a caloric deficit.
Even this approach presented some problems. Assuming you can accurately quantify intake, accurately predicting the amount of calories you use in a day is nearly impossible. There are various charts and tables from various government institutions that say a person my size should consume anywhere from 4000 to 2500 calories a day. So I took the lowest number, and subtracted 700 or so.
So on freshmeat, looking for programs that could track my caloric intake, I found nut. I bought a scale, weighed everything I ate to the nearest gram, and entered it into a nut database. Nut is a great app, after the first week or so, I found that I was avoiding caloric content not because it was good for me, but because I didn't want to corrupt my beautiful data. It was painful going for a while, but nut not only helped me quantify the things I thought I needed to, it also taught me more about the kinds of food I was eating, and what that food consisted of. In doing so, it changed my mind about what I needed to watch out for and why.
Suddenly I wanted to know more about all the types of fat, vitamins, proteins and sugars. I wanted to know what effects these things had on me, and why. So I got me one of these, which I found very helpful. Nut's author also has some interesting things to say on the subject of nutrition. He's obviously a smart guy (judging by his source code alone), and I make it a habit to listen to smart people.