Dehydration often plays a big role in dry lips. Ensuring that you drink enough water on a daily basis can help keep your lips from getting dry in the first place. As our bodies dehydrate, cells of all varieties -- including skin cells like those that compose the lips -- dry out. Replenishing your body with water rehydrates those cells and maintaining a steady intake of water can prevent your lips from drying out in the first place.
So how much water should you drink? The most widely cited rule of thumb is eight, eight-ounce glasses per day. We should note, however, that one 2002 study found no scientific basis for this suggested intake [source: Valtin]. The same paper also didn't find that amount would harm a healthy adult either, though.
On the same token, keeping your environment hydrated can help prevent your lips from drying. Try sleeping with a humidifier in your bedroom.