If rimming your glass with sugar, first cut a lemon in half and run one half along the outside edge of your glass. Next, roll that outer edge in a shallow dish of sugar. Turn the glass upside down to shake out any excess sugar that may have fallen in the glass.
Next, combine the vodka, lemon juice, triple sec, and simple syrup in a cocktail shaker. Fill it three quarters with ice and shake until chilled.
Strain into your sugar-rimmed glass and garnish with a lemon twist, if desired.