DIY: Caulk and Gaps
Referenced from BusinessInsider.com
Problem: Little gaps in windows, doors, or cracks in the walls are letting cool air in, raising your energy bill.
Solution: The Daily Green offers a test to find out where to seal these gaps: Have a friend stand outside the suspected window/door/wall with a blow dryer while you hold a lit candle inside. If the dryer blows the candle out, then it's time to seal those cracks using caulk.
Also check the caulk around your doors and windows outside to see if they've deteriorated over time.