Anonbanon has solid advice that I agree with.
Too many faculty work excessively hard on teaching materials and too little on other tasks that they'd like to focus on (e.g. research). I'm going to be bold and even suggest that those of us at non-R1's (R2's, SLACs) should do this. The key is to develop your teaching materials so that you get consistently high quality student evaluations. Then, only gently revise those materials as you feel necessary. Being at a SLAC, I've found this approach beneficial to my students. By not overhauling my teaching materials each semester (or year) and focusing on research as much as I am able, I've been able to bring many cutting-edge results and theories into the classroom. The students often seem to enjoy this and appreciate how "new" the course content is.
That noted, my suggestions for preparation include 1) try to prep as much course material as you can as soon as you know what you will definitely teach; having to prep stuff at the same time as you teach it gets stressful and drains time. 2) Figure out and start planning (or implementing) an organization method. You're going to be much busier than you expect during your first year. If you do not have a good organization system in place from the start, your office will become a disaster zone and you might feel frazzled. 3) Take a vacation! Even if you don't have tons of cash for a true vacation, try to take some time off. Rest, recharge from whatever stage you're presently at (Ph.D., postdoc) and reward yourself.
Congrats! — My last suggestion is to enjoy the first year. It's a stressful, crazy, and yet incredible sort of time. Take some time each day to sit back and enjoy that you've made it to this stage.