What is the correct usage? I know you sit 'on' a sofa/couch. What about chair?
Piggy-backing on Barrie's answer, you could you say that it depends on how relaxed your posture becomes. You sit in furniture where you lounge, and sit on seats where you sit more upright.
So, I'd be more likely to sit on a bench or ottoman, and more likely to recline in a rocker:
It might also depend on the verb you use, too. Consider that blue divan: depending on how much I wanted to relax, I might either sit on it, recline on it, lounge in it, or lie on it...
Personally, I'd say that you sit in a driving seat, but on an armchair. The reason being that you're surrounded by a cage/frame or other structure as part of the seat. So you're in an aeroplane seat, or in a train seat (as the seats are fixed or designed specifically for that environment) but on a dining chair or on a sofa as they're more "generic" ?