The aim is giving the energy somewhere to go before it overflows, and the things that work are boring on purpose:

Heavy work. Push hard against a wall for ten seconds, carry something weighty across the room, squeeze a cushion between your hands. Muscles working against resistance settles the nervous system more reliably than anything you can think your way into.
Deep pressure. A heavy blanket, or sitting with your back firmly against a wall and your feet flat on the floor.
Longer out-breath. Breathe out for longer than you breathe in. That ratio is the bit that does the work, not the counting.
Something repetitive to hold your attention. A puzzle game, colouring, sorting something. Low stakes, no chat, easy to abandon.
Leave the room. Different walls, different light, floor instead of bed. Changing the input helps more than it sounds like it should.

If someone is with you, less talking helps. Questions during a meltdown add load rather than remove it.