Folded up bits of newspaper shoved in the cracks? Or you can get that plasticy stuff that you attach with a hairdryer to the frame. It's pretty ugly but if you've got plantation shutters, you won't notice it really. You can't open the windows when it's on but shouldn't imagine you'd want to in this weather!
How diy minded are you? Ours were a lot cheaper to do than I'd imagined. We had ours overhauled this summer and they are LOADS better. The beads were removed and draught-proof seals put against the partition beads and down the sides of the beads that were replaced closer to the windows (so they don't rattle at all anymore). I do think you could do it yourself if you are brave enough, though it would probably take longer than it did him. They've got better locks on them too now which hold them much more securely, again minimising the rattle. Took the carpenter a day a window for bay and double bedroom window and then another day and a half for remaining 3 single windows (roughly - he's a methodical worker). He charged £220 a day plus the cost of the beads, seals and locks. (Did other work too - some of the sashes and rails were rotten.) When my dh was a student his parents put a plastic covering across the window frame and hair-dried it until it went tight. Didn't last longer than a couple of months, but did the trick. Otherwise secondary glazing?
we got quoted some extortionate sum for refurb and double glazing of wooden sash windows. We got really unobtrusive secondary glazing instead, plus an overhaul of the windows. the secondary glazing is excellent at keeping heat in.
I'd suggest either secondary glazing which is good but ugle (though ok/unobtrusive) if you ahve shutters and/or getting more quotes for having them refurbished. We did this in our previous house (refurb)and it really worked and wasn't too pricey. Local joiners are often well priced.