An old, more traditional recipe (1930s)

1/3 tequila
1/3 triple sec
1/3 fresh lime juice
Pour over crushed ice
(The ice may be strained out before pouring into cocktail glass).

Juarez variation (most popular before fancy mixtures (margaretatexas site) took over)
1/4 triple sec
1/2 tequila
1/4 lime juice
Crushed ice
Dip wet rim of glass in salt
Strain out ice as the drink is poured into the glass.