Treat Patients the way you would want to be treated if this were happening to you. Everyone likes to be treated nicely when buying an item or going out to dinner. The same attitude that follows good customer service in the retail world (i.e. what keeps customers coming back for more) should be used in this instance. If a patient is being treated well they will be more likely to stand in line without causing problems for staff and/or other patients.
Even though you are a volunteer the patients should still be treated like your best customers. Patients are just as scared as you may be. Ensure that you and your staff are addressing them calmly, respectfully, and professionally.