Bill Tindall, E.Tn.
One such technique is planing corner to corner on a diagonal

The teetering on a jointer bed alternative to simulate this technique was abandoned as soon as I converted an old Stanley into a scrub plane.

Wrong Forum to advocate the following but these diagonals just have to be removed enough so the piece doesn't rock on a flat surface. With this surface down, run through the thickness planer and the top will become planer.

If I could conjure the patience I can see that flattening entirely by hand would be a rewarding accomplishment.