I use equal parts of freshly squeezed lime juice, triple sec, and tequila, plus about ¼ to ½ teaspoon of sugar, depending on the limes, add a bit of water and crushed ice (I don't like so-called "blended" margaritas).
I suppose you could use sugar substitute, but there's still real sugar in the triple sec (or Cointreau if you're feeling like spending money needlessly).