Provide the right training and resources. Employees need the right orientation and training to do their jobs well and be most effective. Ask them what would help and deliver on it. Show part-timers that their jobs matter. Motivate part-timers by showing them how their jobs relate to the company's overall goals.
Communication is the lifeline of any company.
Constantly talk with part-time employees to see if they have questions or concerns. Make them feel a part of the team. Everyone especially part-timers needs and wants to feel a part of the team. Invite them to attend department meetings or off-hours social events. Make it fun.