Stack half of collard leaves and roll into a cigar shape. Cut crosswise into very thin strips (1/16 inch wide). Repeat with remainder.
Mince and mash garlic to a paste with salt.
Heat oil in a 12-inch heavy skillet over medium heat until it shimmers, then cook garlic, stirring, 30 seconds. Add collards with 1/4 teaspoon pepper and cook, tossing, until just tender and bright green, 3 to 4 minutes.