I have a problem most wish they had. We recently restructured our department and kept the top 7 directs in their current positions. The rest of the staff were moved into new roles. I manage the top 7 and they are all high performers. They consistently rank among the top ten in pretty much every aspect of our goals/targets. They are experienced (ranges from 7 - 20 yrs of experience), engaged (most of the time depending on the changes happening in our dept that month and we change often!), and pretty much self-sufficient. I myself have approximately 15 yrs of experience in my field (insurance) but only about 6 yrs of mgmt experience (I say only, because I've not had much direction from my mgr on leading and I've had to learn it along the way).
My directs like what they do (they work in the field) and have no interest in moving into management. They each have a development plan in place and they typically complete them as required. We've recently had some turnover due to demotions but at the management level.
How do I keep them engaged, motivated, and happy to come to work?

Give them freedom
I believe Mark´s answer to this one would be that you don´t have to keep them happy, you have to keep them productive and productive people are happy. Focus on keeping them productive and they will be engaged, motivated and happy. At least that is my interpretation from podcasts I have listened to from Mark and Mike.
And perhaps the key to keep your guys productive might be applying the quote I just learned this week giving advice to PhD supervisors:
"with good students - get out of their way, make contacts for them and remove other obstacles."
The get out of their way part is not actually to be taken literally. You need to be there precisely to identify the contacts they need and the obstacles they need removed hopefully even before they become hurdles to them. Also you need to give them the framework where they should be working (stay within this area) and what they should be accomplishing. But definitely live the how to accomplish it to them and give them as much freedom as you can afford to find not just their path but also where they want to get. Make the framework, the square they must fit in as broad as possible. You might be surprise and they may actually utilize it all, growing your business much over what you expected.
Nara
High performers
Thanks. I've been asking my directs as part of our one on ones more questions along the lines of what tools do they need to continue to be successful. Seems to be working.
Outsource the motivation
I just read an excellent article in the Harvard Business Review on outsourcing motivation. The 2 penny synopsis is connecting your team to the beneficiaries of their efforts. In my case, my team is a group of instructors. I need to find a way to have their students let them know how the student lives have been changed by my team members. For myself, I have a board filling up with pictures of the students and the families of my directs.
In your case, perhaps some of the clients have meaningful stories that can show your team why they are doing what they do.