Throw away any polyester cushion inserts on existing cushions straight away. They never look plump, no matter how much you puff them up. Use either a mixture of feather and foam, 50/50 is always a good mix.
Pipe your cushion, either in the same material as the cushion, we call that self piping, or in a contrasting colour if you want a link to another cushion.
Seasonal cushions change a room dramatically without spending a fortune. Shifting the colour balance of a room can satisfy your winter boredom.
Don’t let your seat cushions look thin and miserable. Memory foam bounces back every time, giving your chair a happy face lift.