I have heard numerous not-really-serious definitions of what makes a gentleman. I can only recall three:
A gentleman is never accidentally rude
A gentleman is never drunk before nine in the morning
A gentleman never strikes a lady without first removing his hat
Anyone have any others?
A gentleman doesn’t “kiss and tell”
A gentleman can play the bagpipes, but doesn’t.
A gentleman takes his weight on his elbows.
A gentleman sleeps on the wet spot.
From “Another Roadside Attraction”: A gentleman never arises from his slumber before noon.
A gentleman always hands his lover a tissue after giving her a facial.
A gentleman never calls his wife a fat cow to her face.
A gentleman always puts the toilet lid down if the flush didn’t quite clean the bowl.
A gentleman never posts nude photos of his girlfriend on the internet until after they break up.
Wear your learning like your watch, in a private pocket; and do not pull it out, and strike it, merely to show that you have one.
A gentleman uses the cuspidor.
An officer shaves twice a day, a gentleman is always clean shaven.
Moved to MPSIMS (from Cafe Society).
twicks, who never worries about being a gentleman
A gentleman takes the blame if his girlfriend farts in public.
A gentleman always seeks permission before wiping his nose on a lady’s sleeve.
A gentleman takes the dishes out of the sink before he pees in it.
[Shamelessly stolen from Steel Magnolias]
You can tell if a man’s a gentleman by how he treats those who can do nothing for him.
A gentleman sends a lady a flower arrangement the next morning. Instead of leaving the cash to buy her own flowers on her nightstand or dresser before he leaves.
A gentleman never uses physical violence on anyone so frail or decrepit that verbal abuse is sufficient.
Should a gentleman make a mark in my hall, I would expect him to clean it.
A gentleman dresses appropriately for any occasion. 
Similar to the one about rude, but a bit more refined:
A gentleman never gives offense unintentionally
re. wet spots:
Gentlepersons put down a hand towel in such a location as to prevnt a wet spot from presenting disconfort
A gentleman must practice sword play at least one hour a day, and fire at least ten shots.