Quality curtains are very effective at retaining heat and are essential in reatining heat, even if double glazing is installed. Ideal curtains are lined (to trap a layer of insulating air between the layers), fall to the floor, and extend beyond the side window frames. The best curtaining options are
3) Roman blinds
Regardless of what sort of curtains you do have there are a few things that will improve their effect
1) Trap air behind the curtain - use velcro sticky dots on the window fram and the back side of the curtain to pull the curtain in close to the frame.
2) Install pelmets
3) Use a channel to keep roller blind close to window frame. The U channel in the photo came from K&D Mitre 10 and was stuck to window frame using double sided tape.