Losing weight is one of the top resolutions made every year, yet only 20 percent of dieters actually achieve successful weight loss and maintenance, says Jessica Bartfield, MD, who specializes in nutrition and weight management.

So what are the other 80 percent doing wrong? According to Dr. Bartfield, there are four main reasons why many dieters fail to lose weight. Avoid these pitfalls and you'll see a svelter bod, sooner rather than later:

Pitfall #1. Underestimating calories consumed

Many dieters watch their calorie intake, which is a good way to keep their weight in check. The problem? Most tend to underestimate the number of calories they eat per day.

Smart strategy: "Writing down everything that you eat -- including drinks and "bites" or "tastes" of food -- can help increase self-awareness," Dr. Bartfield suggests. Pay attention to serving sizes and use measuring cups and spoons to get a more accurate gauge of how much you're eating. And if you frequently eat out, remember that restaurant portion sizes tend to be much larger and much high in calories, so try to look up nutrition information of your favorite take-out meal or restaurant and opt for something healthy.

Pitfall #2. Overestimating activity and calories burned

This is another common mistake dieters make. "Typically you need to cut 500 calories per day to lose 1 lb per week. This is very difficult to achieve through exercise alone, and would require 60 minutes or more of vigorous activity every day," explains Dr. Bartfield.

Smart strategy: A more attainable goal would be to try to increase activity throughout the day and get a total of 30 minutes of moderate to vigorous exercise most days of the week. Buy a pedometer and track your steps; try to increase to a goal of 10,000 steps per day. But be careful -- exercise is not an excuse to eat more!

Pitfall #3. Poor timing of meals

Skipping meals to lose weight is a bad idea. You need a steady stream of glucose throughout the day to maintain optimal energy and to prevent metabolism from slowing down.

Smart strategy: "Eat breakfast every day within one hour of waking up, then eat a healthy snack or meal every three to four hours," says Dr. Bartfield. "Try not to go longer than 5 hours without eating a healthy snack or meal to keep your metabolism steady."

Pitfall #4. Inadequate sleep

"Studies have shown that people who get fewer than six hours of sleep have higher levels of ghrelin, which is a hormone that stimulates appetite, particularly for high-carbohydrate and high- calorie foods. In addition, less sleep raises levels of cortisol, a stress hormone, which can lead to weight gain," explains Dr. Bartfield.

Smart strategy: Aim for seven to eight hours of snooze time every night. Practice 'sleep hygiene' solutions for better sleep: Avoid consuming caffeine, watching TV, exercising or any other stimulating activities before bedtime. Instead, wind down with a relaxing book or soothing music.