A couple of months ago I started dieting with the help of the Fat Secret Calorie Counter, and for me this tool has proved to be very valuable.
Basically it lets you keep track of three things: the number of calories you take in, the number of calories you burn, and your weight.
Intake is measured by choosing from a huge database of ingredients. It really helps that I am single and mostly eat ready-made meals, because calculating the ingredients for home-cooked food and then trying to work out the size of the portions would have been a real downer, I feel.
The list of activities with which you burn calories is much smaller, usually I try and find an equivalent activity from the internet. Luckily the different things I do in a week is quite limited, so I am able to guess calorie usage fairly well.
Calorie counting may not be your thing but a calorie counter can still be a useful just to get a feel for the amount of calories you ingest every day. The rule of thumb is that if something tastes good, it is probably bad for you (at least in the quantities that stores and restaurants serve). Counting calories really brings that fact home.
For me, the Fat Secret Calorie Counter does more than that, it also provides a sort of external motivation. When I dieted before, I tended to get cranky after a week or six—this time around I have managed to keep it up for months in a row, and I have already lost about 11 kilos of the 16 or so I am planning to shed.