Mount drapes, heavy curtains or shades as close to the window possible and be sure to cover the entire area of the window. While this is not exactly a permanent solution like window replacement or caulking, it is still a great option. Consider using dark curtains as they absorb heat.
Sealing with caulking is a permanent solution for closing up the air leaks. If you think you may change your stand on how to seal the air leaks, consider other sealing methods that aren’t as permanent like draft stopper, insulation films, or self-stick weather sealing.
Replacing your leaky, single pane windows with double-glazed windows can help to ensure efficient use of energy.