I think the secret is choosing the right brand in the first place. It is probably fair to say that most brands will lose about 45% of their purchase price over a three year period. One should also consider the price range of one particular model between various "versions" and what extras the Companies have loaded the various models with. Without being able to quote 100% totally correct figures, a Mazda 3 can probably vary in price between say $20K. and shall we say $31K. ?
We have the Honda Jazz that, to all intents and purposes, is highly sought after. We paid $21K. for a genuine Japanese model, but they only gave us $4K. for a beautifully kept Gen.3 V8 Statesman. We easily lost two to three grand. Yet, when I enquired some two years ago with a Honda Dealer regarding trade in value on a Showroom Special HRV, they offered "about 6-7K. on the Jazz. Yet, there are any number of the 2013 model still being advertised for at least $10K., if not more, two years later. Go figure. I think once you're out of warranty, you're up against it, but low mileage and an impeccable service record are a must for top price. One also needs to consider the perceived cost of spares and repairs for an older type of luxury brand - the ones that come to mind are, unfortunately of course, the Europeans, namely Mercedes, Audi and BMW ( both in particular) and then Citroens, Peugeot et al. So, I guess it is a mug's game, as long as you are personally not the mug. Regards Styria