Everyone's body shape is different to some degree, so what's a swell remedy for someone might offer no help to another.
Having said that: from my motorcycling hobby, there are every kind of cushion available: gel pads, AirHawk modular inflatables, foam cushions sculpted every which way, sheepskins, roller beads, you name it. I've used gel pads and inflatable cushions before to cut down on saddle-soreness. (In my case, trying to relieve pain from my hip bone "points" pressing against the saddle/seat).
Back is even trickier. More lumbar? Less? Higher? Lower? I've seen various lumbar solutions for seats, various sliding foam cushions to position wherever.