Engage a tutor. Investigate the tutor option early in the class. Having a tutor doesn't mean you can't handle the class, and you don't have to wait until you get your first "C" to call a tutor. Meet with a tutor less frequently at the beginning of the course, just to reinforce basic ideas and study habits. This way, if you start to struggle, you've already developed a relationship with a tutor who can help you before you become a lost cause.
Start a study group. Study groups are a great way to reinforce what you've learned in class, keep up with the pulse of the class, and gage your standing. Peer pressure will keep you on track with reading and exercises. The more time you devote to engaging in and discussing class material, the better your performance will be in the course.
Meet with the instructor. Remember who hands out your grades. Develop a relationship with your instructor and have casual conversations about the course material. Use the instructor as a sounding board to repeat your understanding of the weekly topics. Let the instructor know that your performance in the course is important to you, and that you're constantly monitoring your understanding of course material.
Read outside materials. AP courses are designed to give students a deeper understanding of and engagement with the material than a regular high school class. In order to grasp many of the concepts presented in AP courses, you need to go beyond the class work and develop your own relationship with the material. When a topic comes up that piques your interest or if you're having trouble digesting a concept presented in class, seek out additional source materials. You can find help at the library or ask your instructor to point you toward appropriate outside reading.