Clean the window frame and glass
Apply weatherstripping around movable sashes
Use caulk to seal gaps and cracks around the frame
Install shrink film insulation over the window
Add thermal curtains or insulated drapes
Use draft stoppers at the bottom of windows
Install interior storm windows
Replace damaged or missing glazing putty
Add rope caulk to temporary gaps
Use insulating window inserts
Upgrade to double-pane or triple-pane windows
Check and reseal exterior trim as needed