How do you keep your medical information private?

HIPAA has helped, but in today's tangled web woven by insurance companies, your medical information isn't really all that private. You can bring a written request revoking your consent to release medical information to the insurance company. (I doubt this works with PPOs.) To be more secure, you can pay out of pocket, not using your insurance.
For more information, see the Privacy Rights Clearinghouse.