A controversial improvement method is to play blindfold chess. Blindfold exhibitions were banned in the Soviet Union in 1930 because they were thought to be a health hazard, though I have yet to see any evidence that this is true. And I’ve found that it has helped my own game to practice visualizing ahead in my mind rather than moving the pieces on the board.
Here’s a recent blindfold exhibition by Magnus Carlsen in which he takes on three opponents simultaneously. A bit of a walk in the park for him, but entertaining nonetheless: