Validate your ticket as soon as you board, before you find a seat

Validate your ticket the moment you board, not after you sit down. On a tram ride home from Lazienki Park I spent the first stops deciding where to sit and noticed a woman near the door watching me with the quiet patience of someone who had seen this before. Nothing happened, but I felt it. The machine is by the door. Do it first, then choose your seat, then look out of the window.