Here's my question: Calculus A & B, for example, is pretty complicated stuff and a lot of very smart parents aren't math oriented. How do they teach a subject that they haven't mastered themselves?
Many of the companies provide cd's with the material on them or homeschoolers network to find competent people to teach certain subjects. Also, many community colleges allow homeschoolers to take these courses from them - some even for free.
Forgot to add...since these resources weren't available to me when I was homeschooling, I hired a tutor for those areas where I felt uncomfortable teaching.
Our curriculum has an online component which included demonstrations for my son's pre-algebra. I'm aware of other curricula which include interacting with a teacher online in real time. In our local area there are also homeschool groups which pay a nominal fee to be tutored as a group by a college student, etc. As the homeschooling communities (and technology) continue to grow, I'm sure we'll see more options develop in future for the more advanced courses.