My answer is pretty simple: come academically prepared and have an effective stress coping strategy.
To academically prepare, 1st, check out the list of required courses and look for your weakest areas. This will, of course, be different for everyone depending on your work and undergraduate experience. For example, I've worked in Marketing pretty extensively, and in undergrad I was a Sociology major, so I felt pretty comfortable with Marketing and Statistics. Because I have written copiously in my career and, as an undergrad, took way more writing component courses than were required and completed an independent study research project, writing and Marketing Research weren't an issue. My areas of weakness however, were Accounting and Finance and that's where I had to work the hardest.
2nd, upon identifying areas to strengthen from the list of required courses, begin to fill in any knowledge gaps now. You can do this by taking a course at your local community college, or, at the very least, pick up a "For Dummies" type of book to familiarize yourself with key concepts - trust me, your future self will thank you! Before starting the program, I took Accounting at Austin Community College, and boy, am I SUPER glad that I did!
An effective stress coping strategy is just as critical to the program. I can't emphasize enough how quickly this program moves - 3 new classes every 7 weeks. But don't be fooled, that doesn't mean the curriculum is watered down - we're being prepared for the national MBA Major Field Test (in 2 days - wish me luck!). Throughout it all, my favorite stress release has been training in Krav Maga at Fit and Fearless (2 blocks from campus!) 3 - 4 nights a week. There's nothing quite as physically and mentally refreshing as punching the heck out of a bag for an hour.
On the eve of starting my 1st year, the best advice given to me came from a dMBA graduate who advised, "Its not rocket science, you just gotta do the work." That summation became my mantra, reminding me that I CAN do it and that, if it was easy, everyone would be doing it. So, as I reflect on my successes and perhaps, even more importantly, my challenges over this 1st year, I'm grateful for the how well the dMBA program has been preparing me to continue forward to pursue my passions.