If you weigh your food, and say, one serving is 30 grams, subtract 10% (3 grams.) Measure out only 27 grams. You will eat 10% less than you are allotted. If you reduce everything you eat, you'll still have a balanced diet, but just a little less of it. It's up to you if you want to enter your food as the exact amount you are actually eating, or if you want to enter it as a full serving even though you are eating only 90% of a serving. This works especially well... (click title for more info)
...once you have lost some weight and need less food to maintain your smaller body. It is easy to do this when eating something that comes in small pieces (cereal, cut vegetables) but not so easy when eating a slice of bread, a small yogurt or piece of fruit, so you may have to pick & choose.