Blackout curtains are an effective way to insulate glass, cutting heat loss by a fourth. Energy film is another, though a bit more expensive insulator. This film has increased transparency and combining its benefits with blackout curtains decreases energy costs significantly.
A very cheap but extremely effective insulator is bubble wrap, which is also very easy to use. Cellular blinds are another good way to significantly increase the insulation value of a window pane. Energy panels provide an effect similar to having triple pane windows, since they insulate by trapping air between the panes, and some manufacturers claim to reduce heat loss by 55 percent.