Keep your average calorie intake equal to your average calorie expenditure (take in as many as you burn). To find out how many calories you need, click here. To find out how many calories are in your favorite foods, click here. Enjoy a variety of healthy foods, exercise regularly, and you will maintain your present weight. Drink plenty of water (8-10 glasses a day is the standard amount). If you are fairly active, a good formula is to multiply your weight in pounds by 16. This number is the number of calories you need each day. If you find you are slowly gaining fat, either add a little more cardio or subtract a few more calories. If you start to loose weight, bump up your calorie intake slightly.
Keep your fat intake below 30%, minimize junk food especially anything containing saturated fat and simple sugars, which are more readily stored on your body as fat. Eat smaller, more frequent meals throughout the day, with less in the evening when you are less active. Try to balance your diet with 60-65% carbohydrate, 15-20% protein, and no less than 20% fat. Thats basically it for maintence. Those who don't need to adjust their weight are usually the ones who know all this stuff anyway.