This is only 2 months in. But these are the things I really wish I'd known before entering.
Start working out if you don't already. Get as much endurance training as you can. Start sleeping earlier, and start eating healthier. If you get a head start on this now, you'll need less nap time and be in a better mood; you'll also, most likely, need far less study time, since every minute you put in will be concentrated, rather than caffeine-driven last-minute desperation.
Get used to bullcrap. I entered d-school thinking that it was going to be a magical land where administrative problems were handled calmly and professionally, professors were somehow different and more professor-y than undergrad professors, and where every day would be building upon my future as a dentist. Lower your expectations: you're going to be squashed through science courses that will repeat your undergraduate biology classes, attendance will be required at certain lectures regardless of whether or not you actually learn well from lectures, and you will have to suffer through classes that seem entirely pointless to you. All this with a hefty price tag.
If you get yourself used to these facts now, and enter dental school with a "That's the way it is" mentality, you will spend a lot less time and energy complaining, and have a lot more time to have fun and focus on what matters, because you understand that's the way it is: get through the crap, benefit from the good stuff. Just remember that even though it sucks pretty hard, it's still a huge privilege to be here, and that just one year ago you would have traded (or perhaps did trade)a testicle to be allowed in.
Figure out how you, personally, study most efficiently. There are going to be tons of students around you who brag about the hours they spend in the library, or how they were up until 3 studying. Ignore those students. The students who spend 5p-8p studying and 8p-10p relaxing before sleeping at 11p are most likely doing just as well, if not better, than those who spend 5p-5a studying.
It's more about learning the marathon mentality than it is "If I take anatomy now will it help me in a year?" Yes, it will. Do it. But don't base your life around prepping for dschool, since the benefits, beyond the establishment of a strong biology background, seem quite marginal. If I were you, I would focus on ramping up your endurance and energy, and put good systems for your personal health in place: you will rarely get a break in dschool, and poor habits or getting sick will make a bad-but-workable place into absolute hell for you.