1 part gin
1 part sweet vermouth
1 part Campari
It's just that simple, but sometimes it takes a firm hand to keep them on track with those equal proportions.
The bibulo.us authors particularly recommend Martin Millers gin and Carpano Antica Formula vermouth for this libation, particularly if you can get Bradley Avey of Bix in San Francisco to mix it.
According to the Negroni's Wikipedia entry, though it is said to have been invented around 1919, the Negroni does not appear in English cocktail books until 1947, but it neglects to mention which book(s) it then shows up in.
According to Paul Harrington in his 1998 book Cocktail: The Drinks Bible for the 21st Century, the name Negroni was given in the 1950s by the Campari company to what had previously been referred to as the Camparinete cocktail and which consisted of
1 oz gin
1 oz Campari
1 oz sweet vermouth
Harrington advocates for an orange slice garnish rather than a lemon twist, to offset the bitterness of the Campari.
David Wondrich's 2002 Esquire Drinks generally agrees with Harrington's assessment, but takes the interesting variant of calling for half-again as much gin (1.5 oz) and an orange peel garnish. Dale DeGroff's The Craft of the Cocktail, from the same year, agrees with Harrington but flames the orange.
In 2006's The Art of the Bar by Hollinger & Schwartz, the recipe is quite changed:
1.5 oz Junipero gin
.75 oz Campari
.75 oz sweet vermouth, preferably Punt e Mes
Flame a wide orange zest and then float it on top