Sue says using bubble wrap windows insulation is rednecking it, which may be true, but it does provide a super cheap and easy way to insulate windows in winter while letting light in too.
This is a simple technique for insulating windows with bubble wrap packing material. Bubble wrap is often used to insulate greenhouse windows in the winter, but it also seems to work fine for windows in the house. You can use it with or without regular or insulating window shades. It also works for windows of irregular shape, which can be difficult to find insulating shades for.
This handy link is from BuildItSolar, who have hundreds of other hints and ideas too. and came via the always helpful Resilient Communities.