The OECD produces readable, well researched studies on a range of subjects including education. The authors looked at cases in eight different countries and at the research literature. They found no magic bullet to increase motivation.
Different things work better in different cultures and with different children.
With that cavaet there were some things that increased the chances of success in motivating young adults. What struck me was the importance of the social framework and beyond a certain basic level the relative unimportance of the quantity or quality of resources. Students in environments where it is assumed they will learn live up to these expectations. Students whose learning experiences are pleasurable learn and continue to learn. Students who have some control over their learning tend to learn how to learn.