What motivates individuals is unique to that person. Start by taking a survey or have some conversations to find out what people get excited about and what makes them work harder. What do they look forward to? It could be incentives like money, prizes, time off, or happy hours.
Another key to motivation is connecting people to the mission. When people are personally invested in the success of the project they are more likely to stay motivated about completing the project. Find someway for them to connect to the purpose of your project or organization. Let people be involved in the planning process so that they understand the route you will take to reach your goal.