1. Keep active.
2. Bring a good hat. While your body can reduce blood flow to fingers and toes to prevent the core from getting cold, for obvious reasons it doesn't want to reduce blood flow to your head. Thus, it's important to keep your head warm. 
3. Eat sufficiently. Your body needs a lot of energy to stay warm. Don't deny it that energy. Mix food with readily available energy (simple carbohydrates: sugar. Chocolate is GREAT for this purpose) and food with slow-but-long-burning energy (fat. My outdoor club's former president mixes quite a lot of butter into his dinner, which he claims helps him stay warm throughout the night)
4. Before you go to bed, run a few circles around the campsite to get warm. It is much easier to keep warm inside a sleeping bag than it is to *get* warm inside a sleeping bag.