Twelve columns, one gutter, everything flush to a baseline. The grid is invisible in the finished poster and unavoidable while making it.
A single grotesque, used at two or three sizes, never more. If a word needs a second typeface, the word is wrong.
One colour, used once, on the largest shape. Red on white is the most efficient signal ever designed.
Nothing is centred except the thing that matters. Centred everything is the same as centred nothing.
Between 1920 and 1932 a group of Swiss designers decided that ornament was a way of avoiding the question of what something was for. Their answer: a poster should be legible from a moving tram, and it should not flatter the person reading it.
The method survived because it is genuinely useful, not because it was fashionable. A grid is a way of making decisions faster, and a single accent colour is a way of making one decision count. The rest is typography, and typography is the whole job.