I agree with Daniel_Burfoot about exercise. 20 minutes of low-intensity cardio exercise is usually enough to increase appetite. Bodybuidlers will often do a small amount of cardio on their off-days to keep appetite up.

You could also eat more foods that are less satiating. Fructose for example (while probably bad for you) doesn't trigger an insulin response and does not contribute to satiety, while fiber-rich foods are more filling. A google search can help you find the right kinds of foods.