Sash Windows

Its important to fill the gaps because a lot of heat escapes that way.
Something like clingfilm could be used and wedged in with a table knife.

If you have several sash windows usually not all of them need to be opened so the film can be fixed to the outer frame which is less work..

The moving panes can be done separately but this takes a bit longer because
the bottom edge of the top window is covered by the top edge of the lower window which prevents access for attaching the tape.
To get round this problem fix the film to the top window and fold back six inches of film at the bottom and fix it using a small piece of sticky tape on each side.
Then slide the top window down and the bottom window up to expose the lower section to allow the double sided tape and film to be attached.

Its also possible to improve the insulation further by doing both the outer frame and the sliding frames so you achieve a triple glazing effect.

Check also the margins on the frame so you can get the correct width of
tape. (15mm or 25mm)