When a new initiative, process change, or system is rolled out, how do you effectively communicate the changes and generate buy-in from your team? What lessons learned can you share that work best?

While the Organization Change Management (OCM) principles are standard for any kind of change, there are nuances based on target audience.  It is hard to discern from the question if you are asking how to help your team adapt to changes being imposed on them, vs help your team to successfully introduce changes they have defined to the rest of the organization.

In general (most of this echos comments from Talbot Smith's reply):

1)  Identify your personas/use cases and determine how the change affects them.  Does an input to their process change, and output, or the process itself?  Will they have to do an old thing a new way, start doing something new, or stop doing something they've always done?  You need to know, for each persona/use case, how the change affects them.  How and what you communicate will differ.

2)  Sell the change in this order:  Why - the reason the change needs to happen.  This is the value the change will bring.  They have to see the value before anything else will matter.  What - the change you are introducing, at a high level.  A new system, a change to the process, etc.  Tie it back to the why.  When - let them mentally prepare for when the change will happen, and give them enough time to provide input if they have concerns.  This is why you start the OCM as soon as the decision has been made to work on making the change.  Lastly is the How.  You'll do this in training shortly before go live.  By the time you get to training (even if it is nothing more than an infographic), everyone should already be anticipating the change and be prepared to understand how to succeed with the change.

3)  Change Champions (what Gladwell's Tipping Point would call your "Salespeople").  You'll need these.  They should be strategic influencers in your organization who can help everyone recognize the value of the change.  Bring them into the project, update them on key milestones, provide them with the critical messaging you want them to share, and measure their engagement.

Small investments in those three steps yield big dividends.

Change management starts on day 1 of any project and continues beyond the roll out of any process or systems changes.  In my experience, it is often necessary to communicate things multiple times before people grasp and internalize the message, so communicate early and often.  Start with the "why". When your stakeholders understand why the change is necessary and what benefits it will bring, they are much more likely to get on board. Follow "why" with "what".  How does the proposed change solve the "why" problem, and why is it the best solution? Help your team make those connections.  What may seem obvious to senior management is often difficult to grasp for the rank and file.  Only when you have buy in on "why" and "what" can you get alignment on "how" - your project plan and timeline.  When your stakeholders grasp all three, you will see a much higher level of buy-in and much of the resistance to change will melt away.  But don't stop there - continue to reinforce and update the message as the project progresses, right through implementation.  And celebrate the successes along the way