What undergrad classes should I take if I want to pursue a graduate degree in I/O Psychology?

I'm currently an undergrad student on track to get a BS applied psychology (which, unlike a psych BA, includes business-related classes like accounting) and I'm currently planning to minor in Counseling and Religious Studies. My plan was to go on a counseling or clinical postgrad route, or possibly MSW. Recently, though, I've been reading up on I/O Psych, and it's starting to seem more appealing to me. If I want to pursue I/O into grad school, what classes should I take while I'm an undergrad?

EDIT: Please include advice for going for a master's in the field as well, since I'll probably end up being sick of school and opting for another 2 years instead of another 5+.