Esquire and About.com both offer good recipes for Negroni cocktails. About.com's recipe works with various types of gin and sweet vermouth, while Esquire's recipe calls for more specific ingredients.
To make About.com's version of a Negroni cocktail, fill a lowball glass with ice cubes. Pour 1 1/2 ounces of any sweet vermouth, 1 1/2 ounces of Campari and 1 1/2 ounces of any gin into the glass. Stir well and garnish with an orange slice or twist of orange peel.
To make Esquire's version of the Negroni, add cracked ice to a cocktail shaker. Pour 1 1/2 ounces of London dry gin, 3/4 ounce of Campari and 3/4 ounce of a sweet Italian vermouth into the shaker. Shake well and then strain into a cocktail glass. Garnish with a twist of orange peel.
Esquire also recommends two similar Campari-based cocktails from Italy for those who prefer a lower alcohol content. To make a Campari Soda, fill a Collins glass with ice. Add 3 ounces of Campari and fill the remainder of the glass with soda water. Garnish with a lemon slice or orange slice. An Americano is identical to a Campari Soda, except that it uses 1 1/2 ounces of Campari and 1 1/2 ounces of sweet vermouth.