Start with a good quality beef broth. If you want to add flavor after the soup has been made, a dash of Worcestershire sauce, barbecue sauce, soy sauce, or miso will add more flavor. Often, just adding more salt and pepper helps a lot. I like to increase the acidity of the soup just a bit by adding a splash of red wine vinegar or a squeeze of lemon juice at the end of the cooking time. You'd be surprised how much difference that makes!