Treat them like a consultant
Start the relationship with a contract to clarify your expectations.
Help your volunteer learn how to be a consultant. Most generalist marketers are employees, with no consulting experience. They know how to do marketing; they may not know how to do consulting. Structuring the relationship will help them adjust to being a volunteer consultant.
Finish the relationship with a close form. This is the way you “pay” your volunteer. It contains your results and your volunteer needs this information for their resume, to build their career. Every marketer needs a resume that contains concrete accomplishments but in a large corporation it is hard to measure the impact that one employee makes.