A technique that I have fallen in love with is the Wim Hof Method. I am going to mainly talk about the breathing technique here but it also includes cold immersion as well. This technique can speed up clearing the mind to a level I cannot explain. This technique includes:
20-30 Deep breaths starting from the belly all the way to the brain. As you get to the end the breath speeds up.
After your last inhale you exhale fully. You are able to hold your breath with a lack of oxygen and this is where you sit in meditation.
Once you get the gasp reflex then take 1 recovery breath and hold for 15 seconds. That is 1 rep, Wim prescribes 3 reps when possible but 1 is a good start.
For more information go to https://www.wimhofmethod.com/