I've been working out since April doing running, push ups, pull ups, core, etc in anticipation of getting Pro Rec Y. In the meantime I've been doing volunteer work as a mountain guide in Colorado and probably going to volunteer with a wolf sanctuary while I wait to hear about final selection. I'll print out the the general orders and basic knowledge packet here and start going over it for a little while each day.
Aside from knowing rank structure for both enlisted and officers, the general orders, and some key people and dates, and getting physically in shape there's not much else you can do to prepare. Getting adjusted to the organized chaos that is probably OCS (only based off what I experienced in USMC boot camp) will be the most difficult part and that becomes normalized after a few weeks of training.