Curtains for winter

[Simplelife]

What are my options for putting up insulating curtains? What type of clips do you get to use the original curtain sliding brackets?
Where would I find them? Or is there a more efficient idea?

[mojo]

Sheets of reflectix cut to fit the windows behind your curtains and blinds.

[bunni]

Refletix works for both heat and cold. Cut them a little larger than the window and squish them in. It will block the light when when the sun comes up too early for you!