Bill is quite right that there is no single solution for motivating students. Service learning projects contain several features that generally elevate student motivation:
-SL is cooperative rather than competitive
-students are active stakeholders
-students can see real-world, tangible outcomes
-it is clear how student contributions connect to a larger picture
-students have a chance to interact with potential role models with the community partners
-different types of students can play different roles, so as Bill pointed out, it's more likely they can find their motivational sweet spot.
We have a page on student motivation that has several strategies and references:
http://serc.carleton.edu/NAGTWorkshops/affective/motivation.html