It takes only three weeks to form a habit, according to obesity researchers at New York Presbyterian Hospital.

That's great news for those of us who find it easier to get started with healthy lifestyle changes than to stick with them. Take one of these small behavioral changes and stick with it for three weeks. Once it becomes a habit, try another one.

1. Drink a glass of water before each meal. How hard is this one? Not only is water delicious and great for your skin, hair and digestion, but it will make you less hungry.

2. Use a few minutes of your lunch break to take a brisk walk each day. Most people who get into the habit of a daily walk—even if it's only down to the lobby via the stairs and back up to your floor—gain a sense of accomplishment.

3. Leave some food on your plate at each meal, even if it's a small amount. I'm talking one bean or a crust of bread at first. Don't worry about wasting food—if you eat it, it turns into "waste" anyway.

4. Replace unhealthy snacks with nutritious snacks. What's your favorite daily indulgence? The afternoon cookie? The evening ice cream? Don't give up everything at once. Just pick one high-fat, high-calorie snack and replace it with a delicious, high-nutrition alternative.

5. Replace one brown food or snack each day with a green food. Brown foods are typically meats, breads and pastries, potatoes, chips, and crackers. Green foods are steamed vegetables or raw greens.

6. Eat something red, purple, orange, yellow, and green each day. Most people are surprised at how monochromatic their diet is. Try it for one day and you'll find yourself saying things like, "Wow, I definitely don't eat enough purples!" Eating a rainbow is the easiest way to get more vitamins and minerals into your diet.

7. Replace baked goods made with white flour with whole-grain versions. Sandwich bread, hamburger buns, crackers, cookies, bagels—everything that is baked these days comes in a healthier whole-grain version. The more fiber, the better.

8. Don't eat after 6 p.m. Studies show that the easiest way to lose weight is to not eat at night. Sip hot herbal tea or seltzer water with lemon wedges in the evening after your meal.

9. Double the time it takes you to eat a meal or snack. It takes most people ten minutes to eat a meal. Set a timer for twenty minutes and make your meal—even if it's just a sandwich—last that long.

These nine little changes can add up to significant weight loss and major improvements in your health over time. Don't try to change too much all at once. Adopt one change until the habit sticks.

Dr. Ro ( www.everythingRo.com) is Nutrition Advisor for The Today Show and Nutrition Contributor to NPR. Named by More magazine one of this country's top five nutritionists, Dr. Ro is America's best-known African American health expert, and the author of the best-selling Dr. Ro's Ten Secrets to Livin' Healthy (Bantam, 2007).