If you find that you just ate under calories for the day, eat a cup or two of Fat Free Light yogurt. Most store brands carry a wonderful selection that will be around 100 cals, and less than 20 carbs (like Publix brand).
And the flavors are great! Instead of eating a piece of cake, I eat Black Forst Cake yogurt, or Carrot Cake yogurt.
There's a reason Michael & Fiona eat yogurt on Burn Notice - it's good for you, cheap to buy, & is great for energy!
I go shopping for yogurt if I even get down to 10 containers in my fridge drawer. I live for this stuff. And it's a great snack at night, and you can add more fruit to it if you like.