Amy in Ann Arbor's Profile

A little trick I read for painting stripes. Tape off the areas you want to remain as they are, then paint across the edges of the tape with the ORIGINAL color. That seals the edges, so the new color doesn't seep under. :-)

A still sheet of air against the window glass should help to block heat, and provide insulation against heat loss as well.

The acousticalsurfaces.com website (cited above) provides custom curtains, but I didn't find prices, and the fact that they have a "financing" link might be revealing.

However, there are also DIY solutions that ought to help with the problem. You might try a standard window insulation kit (~$12 at Ace).

Or, using a thicker clear vinyl sheet might be more effective and easier to live with. You can find vinyl yardage fairly inexpensively at most large fabric stores and/or websites. It comes in a 54" width that should be adequate for all but the biggest windows.

Here is an online example:
http://tinyurl.com/6z9g7j
The price is proportional to thickness, but starts at $5.35 a yard. You can cut the vinyl a bit larger than the glass, and use any non-destructive method to seal it to the casework of the window on all four sides. I would probably try taping it down with Gaffer's (i. e., theatrical) tape or, depending on the thickness and weight of the vinyl, ordinary double stick tape.

With any adhesive method, if the casing is painted, check somewhere inconspicuous that removing the tape won't destroy the paint, and be aware that it may become more solidly "stuck" over time.