There are a few ways to decrease your appetite.
You might want to consider switching to low GI carbs. They release blood sugar at a slower rate so you don't get a sugar spike then crash which makes you end up craving more carbs. As a result they make you feel fuller longer. You can find a list of the GI values of different foods here
Consider increasing your fibre intake. Add some psyllium husks or bran to your morning cereal. Or add a soluble fibre like Benefibre to your coffee. Fibre slows the movement of food through your system so you feel fuller for longer. Just make sure you increase your fibre very gradually and that you drink a lot of water. If you add a heap of fibre to your diet suddenly it can really upset your digestive system. You need to add is slowly to get your body used to it. And you need to drink lots of water to flush the fibre through. Otherwise it sets like concrete in your guts and can make you constipated, which is ironic because most people add fibre to cure their constipation!
Drink lots of water. Often we mistake thirst for hunger. Whenever you're hungry have a glass of water first. If you wake up hungry in the middle of the night water will usually satisfy your hunger.
Go to bed early. I know that I usually get the munchies at 10 or 11pm. The best way to avoid this is by not being awake!
Consider adding some intensive exercise to your routine. Easy to moderate exercise like walking tends to increase your appetite while more intense exercise that raises your heart rate and makes you sweat suppresses your appetite for a while afterwards.
You might want to either have smaller meals more often, or make sure you have healthy snacks available between meals so that you're not tempted to eat high calorie snack foods. Make sure you don't leave more than 3 hours between eating otherwise you'll be starving by evening and it will make you make poor choices in food and potentially over-eat as well. There's nothing wrong with snacking provided it's planned and it's not increasing your fat or calorie intake too much. Currently I'm allowing myself to snack on fruit or veggies between meals.
Add more protein to your meals. Protein lowers the overall GI of your meal and keeps you feeling fuller longer.
Consider cutting out artificial sweeteners. Research has shown that rats eat more after having artificial sweeteners because it confuses the brain about how many calories you've consumed. I'm currently avoiding sweeteners as a bit of an experiment to see if it reduces my hunger.
Hope this helps!