Write down everything you eat. Every snack and every proper meal must be recorded. The immediate effect of doing this is that you're made to think about everything you eat. This alone could help you cut down on the amount of food you consume.
The deeper reason for doing this, though, is that is helps you figure out the emotional reasons why you turn to the biscuit tin.
Do you eat when you're stressed? Maybe when you're feeling a bit down? Or perhaps it's just out of boredom or habit?
Figuring out these triggers is the first step towards taking control of your diet.