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. This way, water slowly evaporates up into the soil structure. And finally, during the winter, get a mist sprayer to moisten the plant's leaves that lose water easier than the soil beneath them.