For nonprofit organizations, activities are sometimes manned by volunteers who do a service for which they usually do not get any monetary compensation. If the cause supported by the nonprofit organization is something that volunteers can be passionate about, then this is compensation enough, as most volunteers are glad to work for experience. Foundations and nonprofits that are meant to help certain interest groups like cancer patients, homeless children and battered wives always have volunteers, because the situations that cause harm to the people involved are also present in so many people's lives. People who felt that they have been helped by others before in their time of need often seek a chance to give back once they have overcome their challenges and darker days.
Most people really care about these causes. For instance, due to increased coverage from the media, features from channels like Discovery and National Geographic and the actual palpable effects of climate change, people are very keen on doing something for the environment. A lot of people are joining in to make sure that their activities are not harming the Earth or any of the animals inhabiting it.
But causes are just mere causes. The only way you can get volunteers is if you can make people know that your nonprofit organization actually exists! A good example for this is through making your presence overwhelmingly common in public places such as malls, train stations and other places where people usually converge. The environmental group Green Peace has been very effective in employing this strategy. By paying for kiosks in shopping malls, putting up posters in parks and doing very carefully planned media publicity stunts, their organization has seeped into the consciousness of many people to the point that they're almost a part of pop culture. This phenomenon has paved the way for millions of people walking in malls or parks to sign up for Green Peace. While not all of them have stayed as committed to the cause as they were during the beginning, the big numbers that they have to start with statistically improves their chances of getting people that an organization can depend on for the long haul.
For small-scale organizations though, the equivalent of Green Peace's strategy is staying relevant and visible in the community. This makes the foundation and the cause easily accessible. For instance, if a foundation runs a service for a local nursing home, the group can host a small social activity in which those with a similar soft spot for the elderly and sick will have a chance to interact with the occupants of a local nursing home.
Volunteerism is a spirit that most people have in theory, but only a few put into action due to constraints in time and decisiveness. By normalizing the idea of signing up and committing a few hours of your time to a cause that you truly care about, you increase the chances of success in terms of making sure that you sustain the volunteer force of your organization.