Sharing a hookah is a ritual with a few simple rules. When you pass the hose, don't hand it over mouthpiece first: fold it back or lay it on the table so the next person can pick it up.
Use your own disposable mouthpiece, even with friends. Don't blow smoke towards people who aren't smoking, and leave the coals to the staff: lifting the foil yourself can easily ruin the bowl.
If the taste turns harsh or weak, just ask for the master. Changing coals or adjusting the bowl is part of the service, and a good lounge would rather fix it than let you sit through a bad session.