Excellent points have already been made, so I’ll echo that yes, you must value the volunteers time. Someone is giving up a day to help your organization- that is important!
Having a volunteer coordinator is a must. There is nothing worse than arriving at a show in the pre-dawn hours ready and willing to help and the scene is absolute chaos. We all know manure can hit the fan, but plan out which volunteer is doing what job before show day. Have written job descriptions, show maps, class list etc ready to go and walk the volunteer through it.
Free lunch, snacks and t shirts are great, but nothing is more important than treating people like they matter.
I think show runners can get in their bubble and forget that some of the best volunteers are non-horsey parents or spouses of riders. Take they time to show them how to do the job, introduce them to other people they will be working with and make them feel welcome make sure that they are confident that they can do the job assignment. Be crystal clear on expectations are, when breaks are, and when their shift is over etc.
Overall - run a good show. Last minute class changes, trainers hogging the warm up, stabling issues etc etc should be dealt with by the show office. Don’t put a volunteer who has no power to fix that issue in the bind. When things go wrong or off the rails don’t blame the volunteer who had nothing to do with it. Be kind and show appreciation.