Yes, the IRS does recognize their role as non-profit volunteers and I'll list some of what they can deduct from their taxable income. They can choose between deducting gas/oil used or mileage. If they don't have their own transportation they can deduct subway, bus, or taxi fare. They can also deduct food and uniform cost, and other expenses they incur and out-of-pocket costs while volunteering.

One of the best incentives for employees is recognition, reward them by communicating face-to-face about how great of a job they are doing. They will continue trying to please you. Another great incentive is to give them flexible hours. Other incentives that worked were giving employees prime parking, pizza parties, flip flop/jeans day, day off work pass and new office chair. Just be creative how you reward your employees and they will work hard. An incentive I don't recommend is cold hard cash.