More advice from firstname.lastname@example.org:
Use your summer vacations for more interesting adventures than lying on the beach with your cousins, especially the summers after 2nde and 1ere. It is better to pursue one or two activities in depth than a lot of activities superficially. Do not be cynical about volunteering. You will be amazed by how much it brings you.
Think long and hard about how you spend your time outside of the classroom. Babysitting? Cooking? Toying with cars? Watching movies? See if you have less obvious activities and use them in your application to give a fair representation of who you really are.