The condition you describe is known as tip burn and generally develops because the humidity isn't high enough around your plants. Most indoor plants hail from sultry, tropical locales, where humidity fluctuates between 60 and 90 percent. In the average centrally heated home in winter, humidity often falls below 15 percent. The most reliable way to battle excessively dry air in your home is by using a humidifier. Group plants together to increase the humidity near the cluster. Other techniques include misting plants daily or placing them on a bed of pebbles in a shallow tray. Add water until it's just below the surface of the pebbles. Keep plants from sitting in the water; they should rest above it, boosted by the pebbles. As water evaporates from the tray, humidity will increase around the plants. You can purchase special humidity trays, or you can use small-cell ice cube trays filled with water to get the same effect.