I've recently been listening to a series of lectures on education by Lisa VanDamme. The CD I've just started focuses on motivation in education. My big take away so far (I'm maybe 20 minutes into it), is that if you provide children with appropriate material presented in an interesting and engaging manner, there is no need for motivating them. Of course, she explains it much better and qualifies things like "appropriate", "interesting" and "engaging."
Here are some pictures of the kids loving what we do and doing what they love.