Marcus's advice regarding AOS and AOC, in Part 3 of his Job-Market Boot Camp, strikes me as highly relevant. I was once told that, as a rule of thumb, an AOS is a sub-discipline which, based on your current knowledge and ongoing research, you will continue to be publishing in, whereas an AOC is an area you feel competent to teach (again, on the basis of your current training and background knowledge). As proof of the former, one would ideally already have a publication record in the area in question; proof of the latter may be a little trickier, but I would imagine that, having a concentration of graduate-level coursework to show for (and maybe having TAed or lectured in the area) would be considered sufficient.
It's important to be realistic about how many areas any one individual can hope to cover: I don't think I've ever seen a CV with more than 3 AOS and roughly the same number of AOCs. -- Finally, as a little 'Easter egg', Marcus's post prompted me to go for a walk in my neighbourhood in Singapore and take a picture of a local TCM (Traditional Chinese Medicine) clinic's advertised 'areas of specialization' -- as a good example of how not to go about advertising one's expertise!