~ Limit your alcohol supply. Provide only a narrow selection of drinks, such as wines, beers, and a few mixed drinks like martinis and gin-and-tonics. Opening up a full, top-shelf bar means the drink tab is going to be enormous.
~ Experiment with wines before the big event. You may be able to find a great vintage at a lower price, so that your guests will enjoy your wines of choice.
~ Arrange with the caterer to have the bartenders open up wine and liquor bottles only as needed, and negotiate not to be charged for unopened bottles. Otherwise, if the bartender uncorks a lineup of bottles and they're not consumed during your event, the bill still goes to you