Obvious answer: by not putting so much food in your mouth.
OK. Laugh. Good fun.
Serious answer: It's not how much you're eating as it is what you're eating. Try to "fill up" on the healthy stuff--fresh greens, veggies, lots of fruits, etc. Don't eat too many overly processed grains and starchy foods like white bread, white flour, and white pastas. Try whole-grain alternatives when possible and couscous can be a nice, healthy alternative for a dinner-time grain that is good for you. Eat a good breakfast with an egg or two for protein--it will keep you full for longer. Never skip breakfast, but make sure it's healthy one. Don't eat a lot of processed desserts; in fact, anything with a lot of "white stuff," like white flour, white sugar, etc is really not good for you at all and will not make you feel full for long. Try snacking on carrot sticks with hummus. Make a salad. Eat whole grains. This is a good start.
Secondly, don't starve yourself. This is counterproductive. You'll just binge later. Eat lots, but make sure its lots of the good stuff. Then you won't feel guilty. Also, make sure you are exercising. Diet is important, but you have to have a holistic approach to your health--whole health.
Hope this helps, and good luck!