I would put it the same way Josh did. Given a few decades (more than likely less time than that), it is pretty much guaranteed to return. If that isn't the case, it'll be the end of the American economic order and your money won't be worth anything anyway. So if you really believe that's going to happen, you might as well just spend your paychecks on hookers and blow.

Dollar cost averaging works if the market goes up-down-up, down-up-down, up-up-up, down-up-up, down-down-up. Pretty much every combination except for down-down-down, which historically, has never occurred.