I have an illness that has affected my mobility and with that posture. So I do physio for this.
When you stand up, lift your chest up. That's all I have to do to get into the correct form. You can suck stuff in, pull chin in, look straight ahead, etc. but that was too hard for me to remember. I used to look kind of odd walking trying to engage everything. So my physiotherapist said "lift you r chest up". When I do that it throws my shoulders back and tightens my tummy naturally. Far easier (I hope that made sense).
The big thing people seem to do now from working on computers is hunching over and letting your chin go way forward. You can try the same thing sitting. Lift your chest up. I stop hunching. When you do it, you'll find you sit differently on your bum (you'll be more centered).
I was told to sit the same way (centered) when watching TV. We tend to slump. If you just sit on the center of your bum (same way you do at a desk once you sit up on your seat bones), you will not slouch on the couch. I need lumbar support (just a pillow) to keep me in the right position.
Hope that helps :)