We use the local high schools. The kids need community service time, so when we have our carnival, we contact the teacher in charge and she has a sign up sheet then she contacts us with the kids' names who signed up. It is greattttt.
Our school uses the fraternities and sororities in our area for free manpower at a number of our events. Three frats even helped us this summer install our new playground equipment! Being greek, I know that most organizations are desperate for community service projects and it really helps us from constantly hitting up our parents. Also, as mentioned in the donation topic line, local community clubs such as the Lions, Kiwanis, etc. are great places to ask for help.