Check out these two videos:

In short, sleeping on your back with the right pillow height and a bed that isn't super soft is generally gonna be the best.

Remember, however, that you shouldn't try to buy your way out of the problem by purchasing all kinds of different external fixes. Ultimately your goal is to build the strength that maintains better shoulder position even when you're in bed.

It may take months or years to build up that strength, but keep working at it!