Refill the sofa cushions. This is the best solution if they are saggy. Filling them will make your sofa much more comfortable. Unless you are experienced, take the cushions to an upholsterer. The filling is generally made of a polyester blend.
Reupholster your sofa. This is the best solution when your couch is uncomfortable due to the material on it. Perhaps choose a microfiber fabric. Just remove the old fabric and have the new fabric sewn on. This is also best done by a professional, but you could do it yourself.
Add comfortable throw pillows to your sofa. To make it aesthetically pleasing as well as comfortable, choose pillows in bright colors and prints. Place three or four of them on your sofa. Place larger ones in the corners to lean against while relaxing on your couch.
Place a soft throw blanket on your sofa. Leave it folded up at the corner of your sofa and hang over the back of the couch. Choose a blanket that coordinates well with your couch.