Cold weather: Wear chapstick. Split lips happen that much easier in cold, dry air.
Don't keep your sparring gear in the car. The cold air makes it hard and it will do less to protect you and your partner.
Warm weather: Drink plenty of water. Either from a drinking fountain or a water bottle. It's very important that your body has enough water.
Don't keep your mouth guard in the car. The heat in the car can cause the mouthguard to become mushy. Then it doesn't fit correctly and can be painful to wear.