This site has an explanation of recommended food portion sizes: Making Sense of Portion Sizes
Weight Watchers also has an article about it: Portion Control
I hadn't heard of the healthy plate that YoYoGirl spoke of but you can follow that rule using a normal plate and just eyeballing values. Make sure half of your plate is steamed veggies or salad, with a quarter carbohydrates and a quarter protein. Filling up on veggies means there's not much room for high-calorie food.
As for snacks, I find that flavoured rice thins are a low calorie, low fat snack with lots of crunch and flavour. They're the perfect substitute for potato chips.