Maybe you're getting married in a few months. Maybe your high-school reunion is right around the corner. Or perhaps you just want to look and feel better than ever. No matter what your motivation, you need to shape up and the clock is ticking. Although there's no diet or exercise program that can change your body overnight, these six tips can get you slimmer and sleeker in only 12 weeks' time.

1. Cut your calories. By shaving 500 calories from your diet every day, you could lose one pound a week--for a total of 12 pounds in 12 weeks. Just don't overdo it: Nutritionists warn that it's dangerous to eat less than 1,200 calories a day. What's more, cutting too many calories could backfire by slowing down your metabolism and forcing your body into "starvation mode."

2. Write it down. According to a Kaiser Permanente Center for Health Research study of more than 2,000 dieters, the greatest predictor of weight-loss success was whether the dieter kept a food diary. In fact, it trumped exercise, age, and even starting body mass index. So be sure to create a food journal, and jot down everything you eat, even snacks.

3. Eat small, frequent meals. When you're trying to lose weight, it may be tempting to wait until your starving to eat a meal. But according to experts, you should do the opposite. They recommend eating smaller meals and snacks every 3 to 4 hours - an approach that will keep your blood sugar levels stable and prevent you from becoming so hungry that you overindulge.

4. Drink more water. Water not only offers a calorie-free alternative to sugary sodas and fattening drinks; according to a German study published in the Journal of Clinical Endocrinology and Metabolism, H2O actually increases the rate at which people burn calories. For weight loss, the researchers recommend drinking two extra glasses of water every day.

5. Go for whole grains. Whole grains are a healthy alternative to their refined alternatives, and they may also help to reduce excess belly weight. In a Pennsylvania State University study of obese adults, researchers found that those who trimmed calories and increased their whole-grain intake shed more belly fat than those who primarily ate refined grains, such as white bread.

6. Step it up. When it comes to rapid weight reduction, exercise is the gold standard. If you don't engage in regular physical activity, now is the time to start, so talk to your doctor about the best fitness approach for you. If you're already active, try to increase the intensity and duration of your workouts to burn more calories.