We may say "every believer is a missionary" but our behaviour proves otherwise. So how do you motivate people?
You don't. You can't.
There is only a small percentage of people ready and willing to take up the challenge of reaching a lost world. Your job is to find them, get them started, keep them encouraged, help them multiply.
They may be new believers or existing believers.
How do you find them? You train everyone and anyone who is willing to learn and then look for faithfulness and fruitfulness.
What about the Majority? Stop trying to change them. It's not working. They won't try anything unless they can see what God does through the Early Adopters.
That means if you are serious about change, all you need to do is influence 15-20% of people and their example will bring others on board. The catch is they have to be the right people, and you may be surprised who they turn out to be. Remember what Jesus did.
Want to learn more? Here's a book that will help.