My trainer used to make me walk around and lay back like I was in a recliner (no hands) then trot in this manner. Then sit straight and canter. That made me sit back and deep in my seat. Another thing is to roll your hips out behind you and center your torso on the spine of the horse. Balance issues seem to cause not sitting deep enough. Have your trainer do a lunge line lesson and explain what you need
What are you looking to improve in your seat? Positioning? or 'less bouncy/floppy'?