"Yes" people will agree to anything, because they want to be liked, but won't necessarily deliver. Asking them what their first few steps will be forces them to think more realistically.
"Maybe" people want to get along without conflict and are torn between the possible advantages and disadvantages of agreeing. Pointing out similar successful projects is helpful, as is making clear that they will have support and will get a big share of the credit.
"No" people see the pitfalls more clearly than the advantages and are afraid of getting things wrong. Acknowledge potential problems and share your plans for overcoming them.
What people have in common is the desire to feel important and valued. If your proposal starts from that position, it will usually be successful.
Jurgen Wolff is a hypnotherapist, teacher, and writer. His most recent book is Do Something Different, published by Virgin Business Guides. His free monthly Brainstorm creativity e-bulletin is available from BstormUK@aol.com