In the UK many museums are adopting a slightly more formal approach because it helps if both parties (the museum and the volunteer) take a little time at the outset to understand the nature of their arrangement. What can the volunteer expect? What support will they get? What will the be doing? It takes skill to match what the museum needs and what the volunteer can offer.
In addition, it is helpful to have an annual review to see if the volunteer would like to take on a different role. Sometimes age or family commitments mean they would like to change, but they find it hard to ask for this – an annual review makes it easier and is a sensible time to do this,. Sometimes the museum has to point out that the person is no longer able to undertake duties they once did or that the system needs to change.
Having a fixed term (which can be renewed) is also helpful to both parties. It can be much easier to find someone who will volunteer for a year or 3 years, than for an open-ended term. It makes it easier for a museum to say thank you and goodbye to someone who is proving to be difficult or unhelpful.
Some museums have also found it helpful to have an “In Case Of” form which can be kept confidential. If a volunteer is taken ill, the form says who should be contacted and how – but it can be kept in a sealed envelope in the museum office until needed. Occasionally volunteers (many of whom are elderly) are taken ill, or even die, in the museum and it is vital to know who should be told.
There are a lot of resources on the South Western Federation of Museums and Art Galleries website http://www.swfed.org.uk/resources/volunteer-framework which I hope you will find useful.