Bubble wrap! Who knew.
Build It Solar has the super-easy instructions, which involve cutting the wrap to the size of the window, spraying a thin film of water on the window, and letting the bubble wrap stick. Add a little glycerin perhaps if you have trouble removing it in the spring.
It's apparently a common practice in the greenhouse industry.
And it's more transparent and high tech than duct taping some foam insulation to the window.
