Pee early & pee often is my motto.

I've found that if I wait until I really need to go right this minute, there's invariably a wait.

Once on an SFO-CDG flight, I was waiting with a couple of other people for the lav. One of the waiters became impatient and started knocking on the door. After the 2nd round of knocks, a rather peeved-looking woman opens the door and said, "Excuse me, I'm doing a medical procedure." She then closed the door and was in there a few more minutes. That once always puzzled me, but it also taught me to be extra patient.