It's easy to augment your existing windows to keep the heat in better, but some "window attachments" are better than others.
Try this little perceptual experiment now: look at yourself in a mirror (or your computer or phone camera). Then look at your left eye, and then your right eye, and then back and forth several times. What do you see?
If someone else is around, ask them to look at you, and look back and forth between your eyes.
What do you see? When you're looking at someone else moving their eyes back and forth, that's exactly what you see. But when you look at yourself in the mirror, you never see your eyes move. When I do this, I can see that my eyes are at a slightly different angle, and I can feel a tiny break in the string of perception when moving them, but I can never see my eyes move.