Best way to improve insulation on old single pane windows?

I've dealt with plenty of drafty old windows and the cheapest fix that actually works is weatherstripping around the frames plus heavy thermal curtains - they trap dead air and make a real difference when you close them at night. If you want something more permanent, window film kits are messy to apply but do help with heat loss. The proper solution is replacing them with modern double-pane windows, but that's expensive, so most people live with the drafts until they can afford it. Temporary solutions beat nothing, though honestly the curtains do more than folks expect.

Window film - the clear adhesive kind - creates an insulating air gap that cuts heat loss without replacing anything. It's cheap, takes an hour to install, and you'll notice less condensation and drafts within days. The previous answer about curtains is solid too, but film addresses the actual glass performance problem rather than just masking it, so you get year-round benefit instead of relying on remembering to close them every night.