"The top mistake I see is not combining their workout with a calorie-controlled diet comprised of healthy foods," Jillian said. She explained that fat is stored energy and calories in our food are units of energy. "Eat more calories than you burn in a day and you will store those calories as fat."

You can monitor your food intake by measuring or weighing your food to ensure your portion sizes are accurate and track your calories in an app like My Fitness Pal or Lose It! It can be eye-opening to see exactly how many calories your meals and snacks add up to and to see what a portion size is. Make sure to track everything, including all those extra bites, handfuls, and add-ons like sauces and dips.

After several weeks of tracking and choosing healthy, whole foods most of the time, you'll have more experience counting calories in your head and won't need to be so strict about tracking every single calorie — you'll just know what works for you.