Caulk and weatherstrip drafty doors and windows
Added
The US Department of Energy says caulk is generally used for cracks between stationary parts such as door and window frames, and weatherstripping is for parts that move, like doors and operable windows. Seal the cracks around door and window frames first.
The result
Result: DOE: caulking and weatherstripping offer quick returns on investment, often one year or less.
Source
US Department of Energy (opens the original source in a new tab). Check it for the latest numbers before you act.