Cut down on some food. If you want just take away two or three things from your lunch. That's a fairly good way.
But an even better way is to take away two things AND do some excercise like maybe running - a mile once or twice a week - or skipping for an hour once or twice a week. This will speed up your motabalism, making you burn calories quicker when you are doing normal stuff like walking.
Another way to speed up you motabalism is to eat your food in small peices, like a snack very hour.
Eating before 3pm can help.