Windows are the weakesk link (thermally speaking) in the average house. They cause more heat gain and loss than any other area of your home - even when they're closed.
To stop this you have to insulate them - double glazing, heavy curtains or some form of secondary glazing that traps a layer of still air. In this GIY project we insulate a timber sash window with a plastic film called Clear Comfort (this is an Australian product but there are a wide range of similar and cheaper products available in the US).
It is affordable (between $5 and 12/m2 depending on the brand), easy to install and really makes a difference.
Trickiness rating: EASY