Reduce the guest list. The largest single elements in your food cost are beverages and the bar. Consider closing the bar during dinner to reduce the number of hours beverages are served. You can also consider an alcohol-free affair. Check with the hotel or catering hall for their prime time.
Midday tends to be less expensive than evening.
Sundays and weekdays are less than Saturday nights. Make certain not to overlap the standard six hour blocks. Reception venues charge extra for daytime affairs that spill over into evening hours. Time of year is also a factor that influences cost. Saturday evenings in December are usually booked for holiday parties.