Every so often, I like to to a prolonged, rigid sit.
This means that for 20 whole minutes, I will sit in a meditative posture, with my spine upright, cross-legged, breathing slowly, not moving a muscle.
(To the best of my ability, for the whole time.)
Don’t look at the timer, don’t scratch your head, don’t even adjust your legs an inch.
Control over the physical body leads to control over the mental, and it takes practice.