My T and I are working on the exact same thing at the moment. I try to "catch" it, before I go too far into dissociation and my T helps me by asking me every now and then.

What has helped me so far:
- changing the topic to something less stressful
- keeping a pillow in my lap (I think it relaxes me a little)
- holding one of those spikey massage balls in my hands. I fiddle with it and if I press down hard enough on the spikes it kinda hurts which brings me back to the present. It also keeps me from destroying my nails/cuticles.
- tossing a ball back and forth
- taking some deep breaths
- standing up, moving my body
- taking a sip of water

My T also has some "tiger balm" (menthol) in her office that she has offered to me before. Apparently the strong scent helps some people. I've never tried it before, though...