I've sometimes read that it's best to develop a plan from the ground up with others so that they "own" the plan as much as you do. This may be ideal, but sometimes its too slow, especially when there are lots of people involved. A friend of mine often seems to want to use the "ground up" approach, and I find that his meetings often don't get very far.
I continue to have the most luck when I make a proposal and solicit responses. I want people to poke at my trial balloon, even if it bursts, because I know that once the proposal changes to incorporate other people's ideas, they start to buy into the plan. I WANT the proposal to change.
These proposals work best when I continuously project that I want input. I have to project that I will not be defensive, that I get excited when someone points out a better approach, even if it means I overlooked something critical or obvious. If I project this right from the start, I think the other people in the room or on the phone can sense that I value their input. It's safe to tell me whatever they really feel. They start to engage and, as the dialogue continues, the plan get better and better and better.