If you find it difficult to find a good posture on a soft sofa, have a look at the Backfriend by MEDesign. It's a very simple product which I find has a pretty much perfect shape to get the right curves in your spine. It's also thin and light weight so is easily portable. The back detaches from the seat, as it can work quite well just using the back section on it's own. The idea is that you can throw it on anything (a sofa, a car, plane or train seat, whatever), so I find it extremely useful. It's surprisingly expensive for what it is, but I haven't found anything else as good.
Alternatively you might want to find a back cushion which is shaped to give you lumber support, which can easily be carried with you to a restaurant or to work.
My wardrobe is full of massage products which have sadly disappointed.
Except for this one which is a permanent fixture in my living room, by HoMedics. It has powerful rotating rollers which kneed your back, moving up and down giving your muscles a good pummelling. They can either rotate round and round to give you a massaging type feel, or just move straight up and down like someone is running their thumbs or elbow up and down your back. There's also a heat function, giving slight warmth through the rollers, and a vibrate function, which isn't very useful so I don't use it. Some models also have extra rollers for the shoulders, but I find these don't help much, so I don't use them (they have a separate control).
Because of all the powerful motors needed for the rollers, it's quite a heavy and chunky unit. It also requires mains power, so you will need a plug socket somewhere nearby, and it's weight means it's not very portable.
Tip: get the model with gel rollers. The rollers on the non-gel version are very hard, and if you're slim without much body fat you may find them too hard - the gel ones have a bit of softness to them. Alternatively drape a thick blanket over it to add an extra layer of softness.
Something which may seem trivial but can make a huge difference. The ideal height for any type of screen, including TVs, is with your eye level at approximately the top third of the screen. This is very important for people with AS to protect the neck and upper spine over extended periods of sitting. Wall-mounted TVs are too often mounted too high. What height is yours?
If you tend to get dull aches in your hip joints after sitting for a while on a sofa, you might find that keeping them moving will ease discomfort.
If this is the case, you might benefit from a fitness / exercise 'mini-bike'. These are small stationary pedals, sometimes designed for an office to go under a desk, designed for exercising your legs while sitting.
They might not work for everybody, depending on your sofa, length of your legs, available space in your living room etc. but they can be great for keeping your hips moving.
You don't have to pedal hard or even get out of breath to feel a benefit when sitting for long periods - all you need to do is slowly rotate the pedals, and this might help to dampen down stiffness and discomfort.
One potential problem if you have carpet in your living room, depending on the pile and depth of the carpet, is the mini-bike slowly sliding away from you as you pedal. To prevent this, check whether the bike is designed for hard or soft flooring, and consider getting an anti-creep mat to place the bike on.
Tip: look for models which use magnetic resistance. These use magnets instead of mechanical rollers to vary how hard you have to pedal, making it much smoother, more reliable and much quieter (essential for watching your favourite TV shows!)