Plan to snack. If you eat lunch at noon, that's at least 5 hours that you have to wait, probably more like 6+. Of course you want a snack.
I try to do a smaller lunch (soup + salad, frozen meal plus something extra, sandwich + fruit, etc (like 300-350 calories)), then plan a protein snack (200 or so calories) in the afternoon - cheese and crackers, humus and veg, a yogurt, another piece of fruit, etc. Also sometimes a hot tea in the afternoon helps. Chewing gum helps, too, if you can do it at your desk.