1. Bring a dish. The official definition of a potluck is "a meal or party to which each of the guests contributes a dish," not a meal or party to which bringing a dish is optional. The latter is called a dinner party. Offering to bring a dish or a bottle of wine to a dinner party is polite, and you should always do that, too. At a potluck, bringing a dish is not polite. It is a requirement.
2. There are no exceptions to rule #1. Even at a potluck -- say, a 200+ person homebrew club party -- where there is a very small fee to offset the costs of supplies (plates, cups) and FOOD (hundreds of pieces of grilled chicken and burgers) because so many people disregard rule #1, you should still bring a dish.
3. Condiments do not count as a dish.
4. When the very nice, crazy generous hosts who have donated their backyard to hundreds of beer-stomping sneakers walk around the party asking for help, you offer to help. If they ask you to grill chicken but you don't want to smell like smoke all day, you grill chicken anyway. If they ask you to cook burgers but you are a vegetarian, you smile and cook burgers anyway, or you find someone else at the party willing to cook burgers for you. You do not simply say you are a vegetarian, turn your back and pour yourself another beer.
5. Rule #4 still applies -- more so, actually -- at a large potluck where there may be volunteer shifts that you may or may not have signed up for (set-up crew, grilling crew, serving crew, clean-up crew). If the lovely, generous hosts come around the party begging for help, you do not sit back and drink your beer and claim you did not sign up for that shift or make up an excuse ("I'm too drunk to help"). You offer to help.
6. When the food begins to slowly trickle over from the kitchen to the buffet tables, a process that due to the sheer quantity of food is going to take 45 minutes because no one has followed rules #4 and 5, you do not jump up to be the first in line. A potluck is not an elementary school cafeteria where "I called it first" tactics preside. Nor do you simply stand in the line (which is now snaking around the corner because others have joined in once they see you make your move), drinking your beer while those half-dozen volunteers spend 45 minutes frantically trying to finish cooking and getting food on the table. You offer to help.