If you enjoy a salted glass, prep the rim of your glass by rubbing a lime wedge along the rim. Dip into margarita salt and set aside.
Shake things up:
Fill a cocktail shaker with ice and add tequila, lime juice, and simple syrup or agave (if using). Personally, I think Dr. Pepper is sweet enough, so it doesn't really need it. Shake vigorously until the sides are fully frosted.
Serve:
Add fresh ice to the prepared glass and pour the contents of the shaker into the glass. Fill the glass with Dr. Pepper, about 4 ounces, to the top.