If you want to lose weight and shape up before summer, there's still time, nutrition experts say.

By Easter, which is March 23, you could drop 5 pounds or more and lose some of your belly fat, which means your pants and jeans would fit better.

By Memorial Day, you could lose 10 to 12 pounds and probably wear a bathing suit a size smaller than one you'd fit into now. And by the first day of summer, June 21, you could drop 15 pounds or more and wear shorts that are one to two sizes smaller than what you wear now.

To accomplish these goals, you'll need to cut calories by 400 to 500 a day, burn several hundred extra calories a day through an aerobic activity such as walking, and start strength training to tone muscle. So says Karen Miller-Kovach, chief scientific officer for Weight Watchers.

Research studies show that dieters lose about 10 percent of their starting weight, an average of 20 to 22 pounds, in the first six months on weight-loss programs. Most of that is lost in the first three months, so it's realistic to start now for summer.

The trick is find a few key areas that are your greatest challenges when it comes to slimming down and focus on them, Miller-Kovach says. "You have to figure out where you're going to get the most bang for your effort."

You might need to cut back on fast food, stop drinking so many calories with sugary sodas or alcohol, eat smaller portions or stop snacking after dinner.

If your problem is late-night snacking, then keep a food journal in the kitchen. Every time you go into the kitchen to grab something to eat, write it down first, she says. It'll make you think twice about eating when you're not hungry.

FIRST STEP, GAIN STRENGTH

If you're not weight training already, start now, Miller-Kovach says. The beauty of strength training is that you'll be toned and stronger, she says. Exercise all the major muscle groups, including the chest, back, shoulders, upper legs, lower legs and arms, so that you'll look better all over, she says.

If you have the resources and the desire, you can go to a fitness center to get started, says Miriam Nelson, an exercise scientist at Tufts University in Boston. Or you can strength train at home with a little equipment and instructions from a book, a video or the Web, or by having a trainer come in a few times.

Beginners should use light weights until they learn correct form, then steadily work up to heavier weights. "Strengthening exercises need to be hard," Nelson says. "You want to rest after about 10 repetitions."

Don't forget you can use your body weight in push-ups, pull-ups, squats and lunges, she says.

STRATEGIES TO FILL UP

When it comes to what you are eating, aim to have 50 percent of your plate filled with vegetables and fruits. Dawn Jackson Blatner, a registered dietitian in private practice in Chicago and a spokeswoman for the American Dietetic Association, recommends retraining yourself to eat smaller portions by using frozen meals, such as Lean Cuisines, once a day for lunch or dinner. Add fresh or frozen vegetables to balance out the meal and fill up.

She suggests eating three meals plus one to two snacks each day on a regular schedule. People who eat regular meals tend to eat about 80 fewer calories per day than those who don't.

Also, sit in a chair at a table and eat from a plate, Blatner says. When people eat mindfully, not standing eating out of the fridge, they tend to eat about one-third fewer calories.

What matters most is your overall way of eating and living, says Dean Ornish, author of a new book, "The Spectrum," in which he suggests that people tailor their diets to their individual needs, genes and taste preferences.

Choose quality over quantity, says Ornish, an internist who is the founder of the non-profit Preventive Medicine Research Institute in Sausalito, Calif., and best known for his program for reversing heart disease.

Ornish also recommends paying attention to how you feel after eating. You'll begin to notice how different foods affect you. More healthful foods make you feel light, clear and energetic. Less healthful foods make you feel heavy, dull and sluggish, he says.

"If you indulge yourself one day, eat healthier the next. As you move towards the healthier end of the spectrum, you're going to look better, feel better, live longer, lose weight and gain health."