The verse I first heard was:

Gentlemen will please refrain
From flushing toilets while the train
Is standing in the station. We love you!
We encourage constipation;
Sherman marched across the Nation.
If Sherman's horse could hold it, you can too.

May I submit this elaboration?:

Gentlemen should try quite hard
To keep their streams within the yard.
Please aim them at the inside of the pot.
There's no need for melodrama,
But the janitor is not your mama.
If you leave marks, a marksman you are not.