The methodology we use for our change management / leadership initiatives is based on John P. Kotter's eight step process of successful change. These steps are clustered in 4 phases:
Phase 1: Set the stage (step 1 and 2)
Phase 2: Decide what to do (step 3)
Phase 3: Make it happen (step 4 through 7)
Phase 4: Make it stick (step 8)
Specifically for organizational change management around rolling out business-critical software solutions, we kept the same 8 steps but we tailored the approach to the specifics of such initiatives. See below schematic of the phases/steps and short description of each step.

Step 1 Establish a sense of urgency
Help others see the need for change and the importance of acting immediately.
Step 2 Forming a powerful guiding coalition
Makes sure there is a powerful group guiding the change. One with leadership skills, credibility, communications ability, authority, analytical skills and a sense of urgency.
Step 3 Creating a change vision
Clarify how the future will be different from the past and how you can make that future a reality.
Step 4 Communicating the change vision
Make sure as many others as possible understand and accept the vision and the strategy.
Step 5 Empowering other to act on the change vision
Remove as many barriers as possible so that those who want to make the vision a reality can do so.
Step 6 Planning for and creating short-term wins
Create some visible, unambiguous successes as soon as possible.
Step 7 Consolidating improvements and producing more change
Press harder and faster after the first successes. Be relentless with initiating change after change until the vision is a reality.
Step 8 Institutionalizing new approaches
Hold on to the new ways of behaving and make sure they succeed, until they become strong enough to replace old traditions.