Include beverages in your party food planning. The amount of punch or number of cocktails or beers a guest will drink varies. Allow for the length of the party, the strength of the beverage, the day of the week, the rowdiness of the crowd, or lack thereof, and adjust your figure accordingly. The rules-of-thumb are:
Punch: Figure 10 people to the gallon-- pretty good mileage. That's a conservative estimate, assuming your guests will drink about three 4-ounce servings during the party.
Cocktails: Figure that your guests will consume 2 drinks per person per hour for the first 2 hours and 1 drink per person per hour after that.