In the morning we expose all our south facing windows to let the light in. That is we pull back all the curtains so there is just glass. When the sun hits the west side of the house we expose those windows the same way. The west windows have, in addition to a heavy curtain that I made (two layers and a sheer liner to soften the light), a roll of mylar coated bubble wrap that I hung at the top of the window and made a latch for so that it can roll up and down inside the window frame thus covering the glass completely. The minute that that sun is no longer filling these windows we close all curtains and roll down the mylar to cover the glass. This holds whatever heat we've gained in the home well into the night.
I put up a couple of photos for you to see the very simple mylar bubble wrap mod I made for the window. I used metal wire to create a ring and a picture hanger hook to latch it to. In steps: cut mylar to fit (snug) in your window. Drill through it and into the window frame where it meets the top of the frame, a few screws with washers will hold it forever. With the mylar rolled up to the top of the window, make a mark with a sharpie where the metal wire latch should come through for it to meet the picture hanger hook. Then open the mylar up and puncture the metal wire through. Hang hook in mid point of window frame, wala! We replace the mylar every couple of years. Stay warm.