"We need our volunteers to do more, but if we ask more of them, we won't have any volunteers."
This is a common refrain I hear from people who run volunteer programs. You can appreciate everything their volunteers are doing, but recognize that they could be doing better or more. Or, perhaps more importantly, you can recognize that for your organization to have a bigger impact, you need more from your volunteers. However, you worry that if you set higher standards or ask for more time from your volunteers that they'll quit and you won't be able to recruit new individuals to take their place.
This is intuitive. But what is intuitive is not always correct.
I have been involved in many organizations that depend upon volunteers over the years, both as a volunteer myself and someone organizing volunteers. During that time, I have discovered what I call the paradox of volunteering: The more you ask of volunteers, the more of them you will get. This is both qualitative and quantitative.
Qualitatively, if you set a higher bar for volunteers, they will likely rise to the occasion. Just because someone is volunteering (i.e. you're not paying them) doesn't mean that they as a person or the work they produce is of less value. They are offering to do work for free because they care about the organization not because their time isn't worth something. Your volunteers are likely highly intelligent, competent, and energetic individuals. View and treat them as such and they will act as such.
Quantitatively, the more you require of volunteers, the more people want to be one of your volunteers. Part of the reason for this is that for most people challenging tasks are seen as worthwhile tasks. Sometimes it's for public recognition, people seeing a person as accomplished. Sometimes it's for personal pride, knowing that they are capable of doing great things. Most often, though, it's because if someone supports a cause, they want to have the greatest impact possible in that cause. People don't have a problem with volunteering more time. People have a problem with volunteering more time when they don't see more of an impact. If you can show that those who volunteer for your organization advance the cause in a meaningful way and that by volunteering more they will have a greater impact, you will have a greater supply of volunteers.
I want to clarify something here: There will be some who cannot commit the same amount of time, resources, or energy as others. This doesn't mean you can ask for the same thing from everyone. Not everyone will be able to dedicate 20 hours/week to your cause. However, if you establish the expectation that some people will put that much in, you lay the groundwork for others to come in and dedicate 10 hours/week or 5/hours week.
If you utilize volunteers, treat them like the intelligent, capable, and cause-oriented individuals they are and work with them to make the greatest impact you can as an organization.