DO-AHEAD TIP: Marinate beef and refrigerate beef overnight (see recipe).
Pierce flank steak with a fork.
In a large zipper-topped plastic bag, combine lime juice, chili powder, cumin and garlic; gently shake to combine. Place beef in the bag; seal and turn to coat; refrigerate overnight, turning occasionally.
Preheat outdoor or indoor grill or oven broiler. Remove meat from bag, discarding marinade and bag. Grill steak to desired level of doneness then transfer to a cutting board and allow it to sit for 10 minutes to redistribute its juices.
Thinly slice beef across the grain (diagonally). Arrange beef slices down the center of each warmed tortilla; spread with 2 tablespoons of salsa and 2 tablespoons of sour cream then top with plenty of chopped cilantro.
Fold and chow down! Serve with additional salsa and sour cream, if you like.