The best way to keep yourself from snacking on junk food during the day is to fill yourself up with foods that will keep you satisfied. There are foods that will fill you up more than others keeping you full for a longer period of time.
Substitute foods that contain white flours for 100% whole wheat flour. White flours are digested a lot quicker than whole ones are because they lack fiber. Any food that has fiber will quickly fill you up and keep you that way. Other foods that contain fiber are vegetables, fruits and beans.
Foods rich in protein will also be digested slowly. Try to consume low fat meats such as chicken, fish, turkey and lean cuts of beef. Foods high in simple sugars such as candy, sodas and other snacks are digested very quickly leaving you wanting more very soon.