In the same way that you would with any other type of exercise, you need to progressively increase the amount of work you are doing in order to keep your body challenged and prevent it from reaching a plateau. Start your workout by climbing a few flights of stairs, and then gradually increase the amount of time you spend doing the activity as well as the intensity of it.
Bringing weights with you or wearing a backpack can go a long way toward making the ascent more challenging. These two solutions will both contribute to the enhancement of your experience.