Recognise that 99% of your workload comes from 5% of the population. The other 95% either support you or don't think about you at all. You'll never contact most of the 95% beyond a simple "good morning" You'll come across the rest every day.
Get education that emphasizes communication and writing skills. You'll use these a hell of a lot more than you'll use say, IT training. Some background outside the classroom with the mentally ill is a GREAT help.
Fitness. It's said that you must exercise 5 hours a week for every minute you expect to survive in a fight. That's true, but I add another fact. Fitness keeps you alive despite the stress and corrosive work experiences. When your buddies start having heart attacks at age 34, this will become VERY clear to you.
Live a model lifestyle 24/7. Dope, drinking, adultery, dishonesty in general, = No job. Keep in mind, you don't become (and stay)a police officer by meeting the MINIMUM requirements, This isn't a cube farm. You become one by out shining out all other applicants. On a related note, choose your friends with care. You WILL be judged by their actions. A cop's life is under a microscope constantly.
If you are married, (or will be) make the career path as clear as possible to her. I've heard MANY a rookie's girlfriend/wife say, "I didn't expect it to be like this".
Lastly, hang around GD. It's a great teaching aid for recognising the 5% and mentally ill.