Graduate programs rarely prepare you for the myriad skills required for being a successful (let alone happy) professor. Most of us pick up these up as we go along; a startling number of faculty never do.
Here’s what I wish I had learned earlier:
1. HOW TO ORGANIZE E-MAIL, PAPERWORK. Most of us learn a slap-dash method of dealing with paperwork while in graduate school and continue these less-than-ideally-organized habits in our jobs. Electronic gadgets can help, but one must have a method of deciding what to save, what to file, how to file, and how to separate material. Efficiency expert David Allen’s system has also helped folks: http://www.davidco.com/about-gtd 
2. SAYING NO/DECIDING WHAT TO FOCUS ON: When untenured, we often say yes to committees, task forces etc. for obvious reasons: we don’t want to displease senior faculty and we need to accumulate items for our tenure file. However, the most successful faculty I know are ones who carefully choose and, even better, find ways to combine research with service, or teaching with research. While you need to be seen as a good citizen, more “selfish” faculty members flourish.
3. MOVING INTO ADMINISTRATION. If you’re marginally efficient and moderately sane, you will be asked to do some serious administrative work – whether it be chairing an important committee, chairing a program, or directing a center. This demands skills rarely accrued in graduate school: leadership skills. While there are some excellent resources and institutes to train academic leaders -- http://fridaycenter.unc.edu/pdep/bridges/  -- most faculty are thrust into these positions with little to no training in how to lead, deal with conflict, or manage their time. Get guidance as soon as you can! Ask a former administrator for a list of important deadlines. Set up regular meetings with an appropriate mentor who can shepherd you through this transition.
4. SELF CARE/HAVING A LIFE: As I have stated in many panels on work/life balance, If you give your life to the institution, don’t expect the institution to reward you with a life. Fight hard for what really matters to your happiness. Sleep, eat well, and exercise. Consider not eating at your desk at least once a week; schedule in exercise. If you’re depleted or ill, you will not teach well or write well.
5. FIND MENTORS. Many mentors. Mentors are allies and you can never have enough. Who around you has successfully negotiated the post-tenure transition? But also look for mentors who exemplify the kind of life you’d like to lead: happy, fulfilled people who don’t involve you in drama or politics.