It may be a little late in the season to begin thinking about insulation, but when I learned about window films I couldn't resist trying them. The windows in our apartment are double-hung, double-pane, and metal-framed--and they look absolutely top-notch. Even so, it is possible to feel a draft around the top of the bottom pane and the bottom of the top pane. Cue 3M's Window Insulator Kit.
The "kit" is just a giant piece of plastic film and a few rolls of tape. After all, its only objectives are to seal any of the window's leaks and to capture a layer of insulating air. That little piece of film can more than double a window's insulating ability. Of course, if windows aren't the main source of drafts, then doubling their effectiveness might not make a difference. But they are and it does (other possible sources are the hole around a ceiling fan, a dryer vent, or even wall sockets facing exterior walls).
Have a look at 3M's Calculator to see what you might save with a bit of film...
But the best bit is seeing the film puff up after you've installed it (I'll try to get a picture). I reckon its the dense, cold air pushing in past the window frame. It's like a cold air balloon. My window films are bulging 3" at the center of the pane (see top picture)... I can't wait to see the difference it makes to the bill.