When you get up from a sofa, try to keep your back straight and use your legs to take the force, don't lean forward and take half of the force through your lower back. If you are aware, you can feel how bad this feels on your lower back and you are hugely increasing the risk of slipping a disc.

When you are slouching on a really bad sofa it may be possible to support your back more with enough cushions behind you, but even then, most sofas sit very low, and the pelvis is often below the knees in height, which is not conducive to a great posture. 

Brentwood Brushed Twill Bedrest
Brentwood Brushed Twill Bedrest

For comfortable back and head support for watching TV while in bed, lean against one of these bedrest pillows at the top of the bed.

Brentwood Originals Brushed Twill Bedrest, Navy

It provides good support while sitting up comfortably in bed.