Exercise physiologist and certified personal trainer Bob Greene has helped millions of people slim down, most recently with his online program TheBestLife.com. The best-selling author shares the sure-fire strategies for losing weight, protecting your health and living your best life.

When you first start working out or following a diet plan, you probably set your sights on a specific goal weight. Maybe you choose that number because it would put your BMI (body mass index) in the healthy category, or perhaps you're hoping to get back down to the size you were before you were married or had kids. Whatever your reason, it's important to set a goal that you can actually reach -- otherwise, you'll be setting yourself up for disappointment.

These four tips will help you aim for a healthy weight: Take a history lesson. What you weigh is largely determined by genetics, so if your parents have always been overweight or obese, you may not be able to get to the lower end of the BMI scale. Certainly aim to be in the healthy category, but taking a look at mom and dad can help you set a realistic goal.

Look at your life. Your habits over time can alter your weight destiny, so assess your past, starting with your childhood: Were you an active, thin child or sedentary and overweight? Then go right through adulthood: What has been the lowest weight you've been able to maintain as an adult? Have you had children? Ultimately, you shouldn't be hoping to drop back down to your high school weight. These questions will help you figure out what a healthy, attainable weight will be for you now.

Evaluate your commitment. Be honest about how much time and effort you're willing to devote to exercise. I don't recommend doing something that you can only sustain for a short period of time because even if you're able to reach your goals in the short-term, if you haven't changed your habits, you won't be able to stay there. Make sure to ask yourself: Can I maintain this for the rest of my life?

Be flexible. Choose a weight range instead of focusing on a specific number. Weight naturally fluctuates during the month and even throughout a single day, due to water retention, hormones and other factors, so trying to hit a certain number and stay there can be tough. As long as you fall somewhere in your healthy range, you should consider yourself successful.