There are a few cultural guidelines toward appropriate dim-sum etiquette which are mainly based on Chinese customs, among them signalling for more tea by leaving the lid of the pot upside down, making sure there are no chopsticks resting straight up in a bowl of rice (a symbol of death and mourning), and leaving a tip for the wagon pushers as a mark of good service.
Dim-sum was originally centered around tea and gradually snacks were introduced. It is now a communal meal and experience shared with friends and family; ordering many dishes to share gives everyone at the table a chance to try everything. Always ask others at the table before taking the last piece of a dish.
Tea is an essential part of the meal. There are usually several kinds of tea available and each one should be fully steeped before serving. Make sure to fill your companions' cups before your own as a sign of respect and humility. To order more tea, it is perfectly acceptable to leave the lid of the pot ajar on the handle as a signal to a server.
Never eat directly from the steamer basket or communal plate on the table - move the item to a little side plate with the main serving chopsticks. If you are having trouble with your chopsticks it is better to ask for a fork or spoon instead of stabbing at the food. And last but not least, never haggle over the check, simply agree on who will pay and then graciously accept to pay for the next dim-sum meal.