Start now, go collect adverts (save them off on the web, as they disappear quickly) for comparable cars, and this is the ammunition you need to present them with, to ask for more money, when they come to you with their 1st offer (obviously if they offer you that value, you take it). Ignore book values, unless it higher than you expect, as it real world sales that count. I couldn’t find exactly the same age car as mine, so collect adverts of cars a year newer and used them (with the proviso, that mine would be obviously worth slightly less).
This doesn’t help when you’ve a great car, with lots of service history & no issues, a tank full of fuel, new winter tyres and a life expectancy of still many years to come.. I was more than a little pissed when that idiot drove into my W reg focus, with a resale value of bugger all.