1.) Liquid Calories
Smoothies and juices are way too easy to guzzle down. Worse, your brain doesn't consider those calories food, so they don't fight off hunger.

Solution: Choose water every other time you're thinking of having a "healthy" drink. Better yet, limit yourself to eight ounces of 100% juice a day and skip juice drinks entirely.

2.) Mindless Eating
Think of it as edible amnesia, a condition where you forget things like mowing through a bag of chips while vegging out in front of the TV, or grabbing a bag of M&M's every time you leave a store.

Solution: Uncover problem times by keeping a record of every morsel (or drink) that passes your lips. Maintain the journal for at least a week.

3.) Inactivity
Even if you're sweating in the gym, you may be moving around less later on — because you're either tired or feel like you've already worked out enough to justify skipping everyday activities such as taking the stairs.

Solution: Buy a pedometer and get in at least 10,000 steps a day — in addition to your workout.

4.) Underestimating Calorie Intake
Blame it on little indulgences. When you cave in and grab a single piece of cheese pizza, it can take 45 minutes of additional cardio to shed those extra calories.

Solution: Just don't eat it. Or save indulging for the foods you truly can't live without — not the mindless junk you don't even enjoy.

See Also:
Five Gym Mistakes
Working out but not losing weight? Here are some reasons