Giving back to your community is a gift that gives right back, and habits and viewpoints instilled at a young age are much more likely to stick with a person than those developed later in life. Because of this, it's great for kids to get used to the idea of being involved in their community through volunteer work and other types of service. Such involvement encourages a generous spirit and helps kids understand that their world extends beyond their own nose.
Let Them Take an Active Role
One great way to encourage kids to volunteer is to involve them in the process. Present them with a problem in your community that they might be interested in: other kids not getting to play sports because their family lacks to means to support it, soldiers who can't come home to their families for Christmas, children in hospitals who are sick and might need some pick-me-ups in their lives; whatever it is you think your child will find the most connection to personally.
Once you've presented the problem, ask what might be done to solve it. Help your child come up with an actionable plan to help others; then enact it! Doing it this way, the child gets to take an active role in volunteering and has a connection to whatever it is you are doing. There is logic to the task and an innate sense of ownership over it. It also shows that you're taking your child's efforts seriously, and that they can better the world in a tangible way.
Lead By Example
The best way to get a kid interested in anything is to lead by example. Parents who regularly volunteer and show empathy to those in their community are much more likely to inspire the same habits in their kids than parents who aren't involved in the community. By demonstrating your own concern and care for others, you model that behavior for your children. Additionally, this is a great way for kids to get "special" time with a parent; if you have made clear that volunteering is a part of your lifestyle, your child will naturally want to be involved in that aspect of your lifestyle.
Foster Internal Motivations
The real key is engagement: find a way to make your child want to participate in the community. If he/she has an internal desire to help, he/she is much more likely to follow through on helping. Whether this internal desire is spurred by connection to a social issue, ownership over a plan of action or desire to participate in a modeled lifestyle, your child will find true enrichment from giving back to others. This enrichment is a lifetime gift that keeps on giving!