So I do hope that if you read this you already know how not to use volunteers; but I didn't feel like I could pass this topic without a few lines.
Crappy jobs- Yes volunteers do a bit of the things we don't want to or have the time to do. But you have to make them understand how it fits into the big picture. You also need to intersperse the not so fun tasks with the fun/exciting/high profile tasks.
The wrong job- Your volunteer signed up to do job or "task X", and you have them working on everything BUT that job/task. That volunteer will leave soon. Take the time to interview and place them in the correct job.
Supplanting- Do not use volunteers to drive down your overhead by letting people go and replacing them with volunteers. Even if you don't have unions, its not the right way to do things. Use the volunteer to extend the reach of your employees. Utilize the skills of your volunteers to make your team stronger.
I can't say it enough that volunteers are valuable, if you do any of the things above, the word will get out and you will have a tough time getting/keeping one of your most valuable assets.