You know what its like – your horse is rather excited at an event or is tense, its not settling through its back while warming up for dressage and you are getting a bit fed up as it refuses to settle.
When this situation occurs – I think, ride like a sack of spuds. I let all my core relax, slouch and relax through my shoulders and arms, and generally ride like a sack of spuds while staying in rising trot. I find this often de stresses you and lightens the mood and will often release the horse through their back once you stop riding like a sack of spuds.
Sadly I have no simple fix for the days when you are riding like a sack of spuds and are aiming not to (and I have lots of those days!)