I struggle with this a lot.
When I can feeling myself leaving and I want to stay present, I'll tell T "I feel floaty". When I just want to leave, I'll let myself go.
T will have me sit up straight (I think it usually happens when I'm laying down or curled into a little ball in the corner of the couch) and put my feet on the floor. I know sometimes I fight him on this and refuse to do it, and just let myself go. But, if I sit up and put my feet on the floor, he'll have me feel the rug under my toes (I always take off my shoes) and describe how things feel. He tries to get me to feel my toes, my feet, my legs, etc. He'll have me look around the room and describe things to him. Sometimes he'll have me drink water.
I know he's had me stand up and hold his hands and tell him things I can see.
Physical touch helps, so he will have me reach across and we'll touch fingers sometimes.
I know I used to just be GONE. Now I am able to stay present so much more, and I'm able to notice myself leaving and do the things T has me do - sit up, feet on floor, look at room, etc - on my own sometimes. When I do it on my own, T usually describes what I'm doing and encourages me: "you're looking at the familiar things on the wall..you're sitting up straight...you're doing a great job"
Therapy is SO HARD sometimes.

