How to Gain Manager Buy-in

Depending on what situation you work in, it can take more than a great idea to succeed. It might take you selling that idea effectively to your manager. When you need your manager to sign off on a new plan, idea, or project, there are a few things you can do to improve your odds.

Sell it. Become a sales person and treat your manager like a prospective client. If you really believe in this project, show it. Present it with power and outline all the possible benefits.
Give them a chance to ask questions. In the process of selling it, encourage questions. You want to hear your manager’s concerns so that you can respond to them appropriately.
Have the answers prepared. I have heard people say that when PHD students go to present their thesis, they should leave an obvious question unanswered so that they can better predict the questions they’ll get and have answers prepared. I’m not recommending leaving anything out of your presentation, but if you can try to figure out what types of questions you’ll get from your manager, you can be prepared to answer them right away.
Calm as many fears as possible. Identify the risks in this project. What if it doesn’t work? What is the worst case scenario? The more you can identify the risks, the more you can show your manager how you can manage them. Managing risk doesn’t mean eliminating it, but it does mean have a plan of attack should things not go 100% to plan.
Can you test it? When you’re doing something brand new for the first time, you can manage risk by testing your way into it. Not all projects allow for testing, but if you can start small and work your way up, especially when it comes to money spent or time committed, it’s helpful.
Involve them in launch. Have a place for your manager to get involved if they want to. Encourage them to become an active player in this new project, which could help allay their fears and make them a strong supporter.
Develop a track record. The more you work on projects that succeed, the more faith your manager(s) will have in your ability and judgment. Keep track of past successes and use them to your advantage.