Building background knowledge is essential as our curricula and our students become more diverse. Look for every resource you can find to help you do that. It's more than just about the textbook.
If you teach history, historical fiction can be a more palatable way for students to gain some basic understanding of people and places from a certain time. You don't have to read an entire novel; choose selections that highlight the background knowledge your students need and read those aloud. Have a classroom library with selections relating to all the major periods/topics you'll be talking about during the year.
If you teach math, check out Bridget Hadley's article on Literacyconnections.com: http://www.literacyconnections.com/StrongReadingComprehensionSkills.php. It outlines how comprehension strategies traditionally taught in English or Language Arts classes also apply in mathematical thinking.
If you teach science, search out biographies on Michelangelo, Marie Currie, Albert Einstein, Dr. Ben Carson and many others. Again, select excerpts that highlight the scientific accomplishments and theories of individuals. Do you have a classroom library? It's one of the best ways to get students engaged in a subject.
If you teach art or music, many of the same ideas apply.
To help you find texts to build those connections, consider picture books. Even if your students are in high school, picture books can be great tools to introduce them to subject areas that they know little about. Check out my newest E Book from Inspiringteachers.com (http://www.inspiringteachers.com/catalog/ebooks/powerful_picture_books.html) where you'll find over 180 picture books referenced that can be used in art, music, history, geography, math, science and language arts instruction.