Another idea might be have have smocks or aprons made in your school color with the logo that can be used/reused for events where people serve food or help others. Not for all events, but for some. I like the name tag too.
We rely on pre-printed namebadges (3"x4") using brightly colored cardstock to identify our volunteers.
While I think the t-shirt idea is wonderful, namebadges are much cheaper and easier to add last-minute helpers to the crew.
Many of our active PTO parents buy the school T-
shirt, and we often encourage our volunteers to wear those at events. However, we have some who don't like to wear T-shirts, can't find the shirt when they need it, or are fitting a shift in between other activities and don't want to wear it or have to change clothes for where ever else they are going.