Heavy drapes. That is all I have to say.
Actually, heavy drapes do help, but, you are now not solving the problem. If the window is leaking air, no amount of fabric in front of it can keep the air from escaping. Fix the window. Get drapes to help. But then, they also don’t let the sunlight in, which helps warm the house. So, maybe only on the North of your house. It isn’t really an exact science, sometimes drapes, sometimes not. This not only works during the winter, but also the summer. It helps keep the warm air out. Sort of. It really just pushing the warm air up to the ceiling. You see, by the time the air reaches the drapes, it is already in your house and is not going to go back the way it came.