With windows closed and the heat on, the winter months can be extra hard on indoor foliage. Here's a simple solution:
Place a humidifier near the plants. For a more low-tech approach, put the planters on top of a tray filled with loose-stones or pebbles and water. The water slowly evaporates up into the soil structure and also adds humidity to air around the plant. During the winter, get a mist sprayer to moisten the plant's leaves. If possible use distilled or filtered water. Tap water contains a lot of minerals and other chemicals which leave spots on the plants' leaves when they dry.