For classic, steakhouse flavor, I don't recommend marinating sirloin steak. Typical marinade ingredients like soy sauce, lemon juice, and Worcestershire sauce will just mask the naturally delicious flavor of this cut. A little kosher salt, freshly ground black pepper, and olive oil is all that's needed. If you'd like to add additional flavor, butter, garlic, and fresh herbs can be added to the pan to enhance the flavor of the basting sauce.