A seam tape like 'seams great' can be sewn into the seam for a bit of movement but a bit of stability too. Any light weight tricot tape can be used. I'm just used to using 'seams great'.'Seams great' is easy to apply and comes in black or white. I keep a few rolls in my sewing stash.
The other process is to stitch along the seam once it's been sewn. On the example below, I've sewn along the back shoulder seam catching the seam allowances and seam tape for a bit of stability.These are the two steps I do to keep shoulder seams in knits stable.