Never pass anyone on the trail without saying "Howdy". 

When approaching someone from behind, give a loud greeting before you get within pistol shot.

Don't wave at a man on a horse.  It might spook the horse and the man will think you're an idiot.  (A nod is the proper greeting.)

After you pass someone on the trail, don't look back at him.  It implies you don't trust him.

Riding another man's horse without his permission is nearly as bad as making love to his wife.  Never even bother another man's horse.

Never shoot an unarmed man.  Never shoot a woman at all.

A cowboy is pleasant even when out of sorts.  Complaining is what quitters do, and cowboys hate quitters.

Always be courageous.  Cowards aren't tolerated in any outfit worth its salt.

A cowboy always helps someone in need, even a stranger or an enemy.

When you leave town after a weekend of carousing, it's perfectly all right to shoot your six-guns into the air, whoop like crazy and ride your horse as fast as you can.  This is called "hurrahing" a town.

A horse thief may be hung peremptorily.

Never try on another man's hat.

Never wake another man by shaking or touching him.  He might wake up suddenly and shoot you.

Real cowboys are modest.  A braggert who is "all gurgle and no guts" is not tolerated. 

A cowboy doesn't talk much; he saves his breath for breathing.

No matter how weary and hungry you are after a long day in the saddle, always tend to your horse's needs before your own, and get your horse some feed before you eat.

Cuss all you want, but only around men, horses and cows.