How can I cure draughty sash windows?

We’re saving up to get them replaced, but are looking for a cheap fix in the meantimeEvery week a Guardian Money reader submits a question, and it’s up to you to help him or her out – a selection of the best answers will appear in next Saturday’s paper.We’ve got lots of old sash windows in our house, which we are replacing slowly and at great cost. While we save up to get the next lot done, does anyone have any tips for dealing with draughty sash windows? What tempo