Yellow leaves on your gardenia could be caused by a couple of different things. It sounds like you've got the water thing under control; gardenias should be kept evenly moist but not soggy or standing in water. Raise the humidity around your plant if you can. Next, I'd recommend fertilizing with a water-soluble fertilizer mixed at half strength every two weeks. Look for a fertilizer with iron chelete, because gardenias can be prone to chlorosis. Chlorosis causes a yellowing of the leaves, especially between the veins.