You probably already know the one surefire way to avoid a hangover. But if teetotalism isn't for you, Fred Freitag, D.O., assistant director of the Diamond Headache Clinic in Chicago, offers this game plan for having your cocktail and drinking it, too. Before the party: Eat, preferably something high in fat. "That helps delay the absorption of the alcohol," explains Dr. Freitag. At the party: Drink up...water, that is. "For every drink, have a glass of water," says Dr. Freitag. Alcohol, he points out, is a natural diuretic, so a long night of drinking can leave you…