Well, there are plenty of ways to keep warm without wasting too much energy! Here are the ones that have occurred to me just now:
1: Keep fit. Working out improves your physical and mental health, and, the more muscle you have, the less likely you are to catch cold!
2: Have tea with honey and lemon in it. Make large quantities at a time and store it in a thermos, so you don't have to boil hot water every time you need a cup.
3: Drink alcohol if you have a good tolerance! A shot of decent liqueur will warm up your throat in seconds.
4: Eat soup! Veggies are so delicious and nourishing. Soup will keep your body very warm and healthy (if possible, use fresh vegetables for your soup rather than frozen ones and avoid ready-made crap at all costs! I'm willing to bet those contain zero nutrients, and think they taste disgusting!).
5: Use common sense. If you're out all day, turn off the central heating. Only use it while you're in the house when you feel it's necessary. Keep doors closed, so heat doesn't escape.
6: Dress appropriately! Remember that natural fibres (eg. cotton) will keep you warmer than acrylic ones and limit perspiration!
7: Eat properly. Home-made food is considerably healthier than street food, and keeping an oven running will keep the kitchen warm.
8: Have baths/showers with your sweetie! This will cause you to bond and save water!
9: Cuddle up to your sweetie! Grab a blanket, get on the couch and spoon! It's a guy's job to take care of his girl, which includes keeping her warm!
Hope these helped or atleast amused you!
Me, myself, my common-sensed-self and I.