Ensure that the climate in the classroom is welcoming of all students. This can include providing students opportunities to discuss ideas with their peers, making sure that students know how and where they can get help, and not equating experience with "smart."
Example: "We are happy you decided to take CS! You belong here!"
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-recruitment-in-HS