Dress appropriately. No flip-flops.
No gum.
Stand up straight, and smile.
Have a passion for volunteering, because otherwise it will show.
Work hard and do your best.
Be kind: no teasing or bullying. Respect other peoples' opinions.
If you make a mistake, try again. Be honest if you do something wrong.
If you are in some form of food production and you have long hair, tie it back.
Keep your hands clean, and follow basic principles of hygiene such as not sneezing or coughing on other people.
No chatting.
I chose to add that a volunteer should dress appropriately, as that is respectful to clients, customers and other volunteers. It will also mean that you are wearing correct clothing for example: if you work with bees.