Our volunteers are generally aged 18 - 30, so we want a style that works for them but also wouldn't be out of place on older volunteers. We would like the style to work well with our logo, using the same colours. The t-shirt needs to work well outdoors, so designs that work on darker colours like navy or black, or on natural coloured t-shirts, are preferable to white backgrounds. It will be used for our programs in both Australia and New Zealand, so we would like to avoid anything too country-specific unless you have a great idea on how we can do this!
Our volunteers see themselves as just that - volunteers, so we would be interested in a bold design that tells everyone they are a conservation volunteer. They are hands-on conservationists, so you might also consider using graphics like garden implements (spades, shovels, rakes etc), work / garden gloves, or work boots - these could be in addition to 'volunteer' or instead of it.
You might also be interested in trying out some off-the-wall slogans such as:
... or anything else you can think of! These are examples only - you certainly don't need to use them if you don't want to.
Our strapline is 'Connecting people with our environment' so you may wish to incorporate this instead of a slogan.
Our website at http://www.conservationvolunteers.com.au will give you more details about our organisation.