I dissociate a lot under stress as well. I've found keeping my hands busy helps (coloring, playing with fidget toys, etc). I also respond to cognitive tasks (like guilloche was describing: talking about things that interet me, lighter conversation, switching the subject).
Today T tried a relaxation exercise, but it ended up being triggering (just so happened the guided mediation she picked had three elements from what had been triggered during the session).
dissociation is a fear reaction, so if you can find a way to maintain a feeling of safety in session, it should help curb the dissociation.